Fixes #126

TL;DR
=====
This makes it possible to read and write snappy compressed streams that
are compatible with the java and scala kafka clients (the xerial
blocking format))

Xerial Details
==============
Kafka supports transparent compression of data (both in transit and at
rest) of messages, one of the allowable compression algorithms is
Google's snappy, an algorithm which has excellent performance at the
cost of efficiency.

The specific implementation of snappy used in kafka is the xerial-snappy
implementation, this is a readily available java library for snappy.

As part of this implementation, there is a specialised blocking format
that is somewhat none standard in the snappy world.

Xerial Format
-------------
The blocking mode of the xerial snappy library is fairly simple, using a
magic header to identify itself and then a size + block scheme, unless
otherwise noted all items in xerials blocking format are assumed to be
big-endian.

A block size (```xerial_blocksize``` in implementation) controls how
frequent the blocking occurs 32k is the default in the xerial library,
this blocking controls the size of the uncompressed chunks that will be
fed to snappy to be compressed.

The format winds up being
|   Header    | Block1 len | Block1 data  | Blockn len | Blockn data  |
| ----------- | ---------- | ------------ | ---------- | ------------ |
|  16 bytes   |  BE int32  | snappy bytes |  BE int32  | snappy bytes |

It is important to not that the blocksize is the amount of uncompressed
data presented to snappy at each block, whereas the blocklen is the
number of bytes that will be present in the stream, that is the
length will always be <= blocksize.

Xerial blocking header
----------------------
Marker | Magic String | Null / Pad | Version  | Compat
------ | ------------ | ---------- | -------- | --------
 byte  |   c-string   |    byte    |  int32   | int32
------ | ------------ | ---------- | -------- | --------
 -126  |   'SNAPPY'   |     \0     | variable | variable

The pad appears to be to ensure that SNAPPY is a valid cstring, and to
align the header on a word boundary.

The version is the version of this format as written by xerial, in the
wild this is currently 1 as such we only support v1.

Compat is there to claim the minimum supported version that can read a
xerial block stream, presently in the wild this is 1.

Implementation specific details
===============================
The implementation presented here follows the Xerial implementation as
of its v1 blocking format, no attempts are made to check for future
versions. Since none-xerial aware clients might have persisted snappy
compressed messages to kafka brokers we allow clients to turn on xerial
compatibility for message sending, and perform header sniffing to detect
xerial vs plain snappy payloads.