Add lz4 compression type

[vmarkovtsev]

Fixes #254

Using python-lz4 here. I had to implement the custom stream wrapper as we read the file in blocks of fixed, small size and need to accumulate them. Unfortunately, lz4 does not contain the stream implementation yet.

Additional notes:

• The default block size is in bytes now, not in numpy array rows (wtf? I guess it was a bug), and I set it to 4 MiB. Does not change anything for zlib and bzip2 (we use streams), but improves the efficiency of lz4.
• I increased the data size in test_defragment(), otherwise bzip2 does not pass - likely because of the headr overhead.
• I allowed compression objects to miss the flush() method.

[drdavella]

This introduces lz4 as a test dependency. Can you please update setup.py so that this dependency is resolved and the test passes?

[coveralls]

Coverage increased (+0.07%) to 94.229% when pulling 41e6398 on vmarkovtsev:lz4 into ab99df1 on spacetelescope:master.

[vmarkovtsev]

@drdavella Done. I had to add lz4 to travis and appveyor configs. As for setup.py, added lz4 as the optional requirement in case "CI" env is defined (Travis). lz4 tests are skipped in case lz4 pkg is not installed.