Support signed int as return value.

[tushushu]

User can decide whether use signed int(i32) or unsigned int (u32) as hash32 function return type. For example:

>>> import mmh3
>>> mmh3.hash('foo', signed=False)
4138058784
>>> mmh3.hash('foo', signed=True)
-156908512
>>> mmh3.hash('foo')
-156908512