Misc bugfixes and increased test coverage.

Some usages which would segfault now just raise NotImplementedError.
Fixes #44

Fixed bug in the BloomFilter.copy method.  Fixes #43

Fixed initialization of in-memory bit-vector when not using a file/mmap.  Fixes #46

The constant, ReadFile is now available in Python as
pybloomfilter.BloomFilter.ReadFile

bf_from_base64 and bf_from_file were replaced with BloomFilter class
methods, from_base64 and open, respectively.