Refactored project such that top level hdlmake directory is now a package. Added setup.py for easy packaging of project with setuptools. Note that installation instructions (and how to run hdlmake) has changed!

Related changes:
- hdlmake/_version.py file now exists as a single location to manage the version of hdlmake. This is used by Sphinx docs, setup.py, and hdlmake itself.
- Cleaned up some circular dependencies in hdlmake.fetch package. This changed the design of backend_factory such that it no longer depends on importing git/svn fetchers.
- Added tests/run_tests.py. Right now it has one test which iterates through all of the top level sim and syn Manifest files and attempts to run hdlmake against them. This acts as a crude regression test.
- Added ez_setup.py. This installs setuptools if it is not available in the python environment. This guarantees that setup.py will work.
- Added Manifest.in. This lists non-hdlmake application data that should be included in a source distribution (documentation, t...