Pyfastx

- Fixed large offset became negative error
- Fixed slice step
- Fixed uncorrect median length
- Fixed strand compare error
- Added GC skew calculation
- Updated test script

- Changed int type to standard type
- Added support for processing large fasta file
- Added id number for each sequence
- Fixed SQL fetch error
- Used 50 as default value of nl to calculate N50 and L50

- Added support for searching subsequence from a sequence
- Added support for checking subsequence weather in a sequence
- Fixed gzip index import error
- Fixed subsequence parent length for full sequence extraction

- Fixed unicode error caused by sqlite3_finalize

- Fixed seq description unicode string error

- Fixed sequence description decode error