Fix tests for compatibility with pypy

The pypy garbage collector can not be relied upon to be run immediately after
the deletion of the last hard reference to an object. Our tests previous relied
on this implementation detail of CPython, which is not part of the
specification though. Therefore, the garbage collection has to be run manually
to ensure proper execution of the tests in pypy.
Reenable pypy tests in Travis CI

Since the tests have been fixed for pypy, continuous integration for that
platform can be reenabled.
List supported platforms in readme

The Travis CI build ensures that the module works on different platforms. They
are now listed in the readme file.
Merge branch 'topic/pypygc'

This branch addresses the compatibility problem with pypy raised in issue 6,
see https://github.com/aepsil0n/obsub/issues/6 . pypy is now fully supported by
obsub.