Change default alloc size for uncompressing to 16K.

Remove comment about core dumps when an invalid window sizes is used.
This bug has been fixed in zlib 1.0.4.

Two new optional arguments to decompress, wbits and bufsize.  wbits
specifies the window size and bufsize specifies the initial output
string size.

In decompression code -- decompress and decompressobj methods -- use a
Python string (and _PyString_Resize) to collect the uncompressed
stream.  Replaces a separate buffer that was copied into a string.

Fix bug in decompress that caused it to always realloc the buffer when
it was finished decompressing.

Modernized handling of optional arguments to compressobj.

Updated doc strings.