Only remove the site.pp if it is empty

The rspec-puppet documentation mentions than any node definitions for node tests should be placed in spec/fixtures/manifests/site.pp, but this file is uncontitionally removed by `rake spec_clean`. This commit changes that behaviour to only remove site.pp if it is empty.
add backwards-compatible support for arbitrary git refs in .fixtures.yml
add opts logic to rake spec_clean
Fix Mocha deprecations
Merge pull request #24 from hunner/dont_kill_site

Only remove the site.pp if it is empty
Add readme for fixtures
Merge pull request #27 from hunner/add_git_refs

Add backwards-compatible support for arbitrary git refs in .fixtures.yml
Merge pull request #26 from mitchellh/fix-mocha-deprecs

Fix Mocha deprecations
Rake should fail if git can't clone repository
Merge pull request #28 from hunner/fail_to_clone

Rake should fail if git can't clone repository
Updating CHANGELOG for 0.4.0-rc1