Move FileList generation to rake tasks

Gives us a single `#check` method to unit test. Though I'm not entirely
happy about the layout of these classes. Added pending spec tests for the
rake tasks, which will need to test whether they find the correct files.
Render all errors and warnings to strings

This will make it easier to test the ouput of the `#check` methods and
prevents any voodoo from happening when the array is joined into one big
string by the rake tasks.
Don't catch StandardError for templates

I can't see any case where StandardError (which actually encompasses quite a
few exceptions) would be raised by a template. We're only generating
SyntaxError. Should this ever occur it will raise an uncaught exception and
abort the rake task, so we'll find out about it.
Add a helper method for loading fixtures

I'm not very happy with this approach. I'd prefer to have the Puppet and ERB
code inline with the `it{}` blocks and written to temporary files. However
this is a suitable MVP.
Implement tests for manifests

Convert all the existing manifest tests from pending.
Implement tests for templates

Convert all the existing template tests from pending.
Move all fixtures to a Puppet module layout

This will come in handy when we want to test that FileList is doing the
right thing. We can just point it at the fixtures directory.

The helper methods had to be adjust accordingly, to add a `manifests/` or
`templates/` directory to the path.
Test for non-array items passed to #check

Both check methods expect an array of files to be passed. Test for an
exception if something else is passed in.