Streamline "r", "rw" and "rw+" (create) operations

[mizvyt]

Regression: removed "mode" parameter from BloomFilter.from_base64() introduced in 0.5.0 (https://github.com/prashnts/pybloomfiltermmap3/pull/13)

I had doubts about the existing flow in pybloomfilter.pyx, so hopefully these changes will clear things up a little:

• By creating a BloomFilter instance, we expect a new file to be created by default (or done in-memory). No mode argument should be supplied.
• By calling BloomFilter.open(), we expect a file to already be present for reading, thus allowing to specify the file read mode (either "r" (read-only) or "rw").
• By calling BloomFilter.from_base64() / BloomFilter.copy() / BloomFilter.copy_template(), we create a file implicitly and open it in "rw" mode.

[mizvyt]

With 9a360ce I've changed references to the old bf.name.decode() to bf.filename as of #25. Rest of the commits were a result of a rebase. Merging this for 0.5.1.