Save memory when decompressing data

[dachengx]

Streaming decompression can significantly reduce memory usage of bz2 and lz4. To keep backward compatibility, the decompress key is kept. The memory-reduced function is assigned to _decompress key.

[coveralls]

coverage: 89.184% (+0.06%) from 89.122%
when pulling 89919a4 on low_memory_decompress
into 8642c39 on master.