Add test suite

This requires Python 'requests' module[1] and uses the standard 'unittest'
module. It is a black box test suite: nothing is tested internally, only how
the server reacts given certain inputs.

It assumes lwan is built in the 'build' directory, and that
'build/files_root' points to 'files_root'.

[1] http://www.python-requests.org/en/latest/