restructure API documentations
make core.py as module nextbiopy.core

Move basic elements in core/base.py, more structured now.
suppress flake8 error on __init__.py
add FASTA readability in new module nextbiopy.io

This will be the first/simple try for issue #12, using pure Python
solution. Further functionality like writing or appending to FASTA file
should be implemented in near future.

For C implementation, I will put them in nextbiopy/io/_cfastaq.py
instead, using a separate branch.

Documentation ... Ummm, see you next commit T__T
update FASTA documentation
update Seq documentation
goes to version 0.0.2