migen/test: start unittests
test/test_fifo, genlib/fifo: move test to unittest
test/test_fifo: do not use relative import

This way the test can simply be run with:
 python -m unittest test_fifo.py
genlib/coding, test/test_coding: unittests
genlib/coding: add docstrings, add it to api doc
setup.py: update repository address