THPPlugin calls Python API without GIL

THPPlugin calls into the Python API without holding the GIL. Most of the calls look benign (read-only access), but it probably should be cleaned up:

``` python
...
        Py_UNBLOCK_THREADS;
        THTensor_(set)(LIBRARY_STATE ((THPTensor*)self)->cdata, ((THPTensor*)PyTuple_GET_ITEM(args, 0))->cdata);
        Py_BLOCK_THREADS;
...
```
