use Py_ssize_t in decompress_ptr

Since we are using 's#' here, we don't need to check if cbytes is less than
zero, since the length of the corresponding python bytesoby can never be less
than zero.

Adresses #26
fix return type of decompress_ptr

Adresses #28 and decompress_ptr now returns the number of bytes written to the
buffer.
expose returning nbytes from decompress_ptr to toplevel
Merge branch 'fix_decompress_ptr' of github.com:esc/python-blosc into esc-fix_decompress_ptr