towards Python3 compatibility
Removal of the primer base is done in its own modifier class.

This simplifies the code a bit and may even by slightly faster.
Make writing FASTA/FASTQ a method of the Sequence class.

This improves writing out reads by a few percent.
Implement quality trimming in Cython.

This speeds up quality trimming fourfold. Total runtime goes down by ~10%
if nothing else is done (no adapters trimmed).
Do not use bytearray in Python 2