Add a StringMatch object, which represents a matched string. It has an
identifier member (this is the string identifier, eg: $a) and an instances
member which contains a list of matched string instances.

It also keeps track of the string flags internally but does not expose them
directly as the string flags contain things that are internal to YARA (eg:
STRING_FLAGS_FITS_IN_ATOM). The reason it keeps track of the string modifiers
is so that it can be extended to allow users to take action based upon certain
flags. For example, there is a "is_xor()" member on StringMatch which will
return True if the string is using the xor modifier. This way users can call
another method (discussed below) to get the plaintext string back.

Add a StringMatchInstance object which represents an instance of a matched
string. It contains the offset, matched data and the xor key used to match the
string (this is ALWAYS set, even to 0 if the string is not an xor string).

There is a "plaintext()" method on the StringMatchInstance objects which will
return a new bytes object with the xor key applied. This allows users to do
something like this:

```
print(instance.plaintext() if string.is_xor() else instance.matched_data)
```

Technically, the plaintext() method will return the matched_data if the xor_key
is 0 so they don't need to do the conditional but this allows them a nice way to
know if the xor_key is worth recording along with the plaintext.

I decided not to implement richcompare for these new objects as it isn't
entirely clear what I would want to do the comparison on.

Add a "matched_length" member to match instances. This is useful when the
"matched_data" member is a subset of the actually matched data.

Add a test for this that sets the max_match_data config to 2 and then checks to
make sure the "matched_length" and "matched_data" members are correct.

Add support for getting the list of available modules. It is available just by
accessing the yara.modules attribute, which contains a list of available
modules.

>>> print('\n'.join(yara.modules))
tests
pe
elf
math
time
console
>>>

Note: This commit also brings in the necessary defines to build the authenticode
parser, which is also done in the xor_value branch. Also, this commit updates
the yara submodule which will likely overwrite the changes done in the xor_value
so I recommend updating the submodule after both are merged.