Do not run Cython by default.

Cython is still run if:
- the --cython command-line option is given
- or the pre-compiled C files are missing.

Not running Cython by default is recommended by the Cython developers and
should reduce problems people had when installing cutadapt.

The code is partially taken from 'cytoolz':
https://github.com/pytoolz/cytoolz/blob/master/setup.py
move rest_writer to AdapterMatcher
refactor

everything is now a modifier, including adapter removal and quality trimming
move most modifiers to separate module
apply -> __call__
Use cythonize() function.

Recommended way to describe Cython extensions.