Support 'key in bloom' in Python wrapper.

This is a more Pythonic usage than bloom.check(key), normally
implemented via a __contains__ method for a class written in
Python, but here via the sq_contains slot in the C code. Note
this implementation Only looks for strings (anything else
returns False).
Merge pull request #50 from peterjc/py_contains

Support 'key in bloom' in Python wrapper.
tweak some Makefile variable names to be more conventional

rename:
PREFIX -> prefix
LIBDIR -> libdir
INCDIR -> includedir

refactor CFLAGS and LDFLAGS to better work with user preferences:
ALL_CFLAGS and ALL_LDFLAGS for flags that are probably required
replace LDLIBS with ALL_LDFLAGS
SHARED_LDFLAGS for building the shared library
use ALL_CFLAGS everywhere CC is used, including for linking
test tweaks: separate failures from warnings, exit with error if any failures
Merge pull request #56 from ploxiln/makefile_conventions

Makefile tweaks to match some conventions
offset should be type long, not int
Merge pull request #55 from lilydjwg/master

Use type long for offset