Implement Draft#url.
Add Site#conflicts method.
Raise PostConflictError when generating on any conflicts.
Shift bin/serif logic into Serif::Commands.

Let's us test it more thoroughly.
Add some tests against Commands.
Test that serif -h|--help exits.
Assert that generate_site calls Site#generate.
Fix rspec warnings about subject in :all blocks.
Quieter draft_spec.
Let 'serif generate' capture conflict errors.
Catch site conflicts in the admin interface.

Very hacky, but it'll do.
Replace Post.from_slug with Post.from_basename.
Replace admin post edit URLs with basename, not slug.
CHANGELOG about generation conflicts.
Merge branch 'master' into name-conflicts

Conflicts:
	CHANGELOG.md
Merge pull request #47 from aprescott/name-conflicts

Post URLs may conflict