Cleanup file.

Hoe is going away. Manifest.txt is generated by Hoe, so removed.
Gemfile for Bundler.
New Rakefile and first Rspec tests.

- Rakefile loses anything related to Hoe.
- Rakefile now solely runs Rspec tests, for now.
- A couple of trivial tests with Rspec, to get started with the
  migration.
Added a few basic tests for Filter.
A couple more spec for filter.
Run existing test suite with Rake.