Gem refactor

Nat,

I did a small refactor of the gem to follow conventions such as:

- removing the lockfile
- separating out runtime and development dependencies
- gem structure - a single entry point at `lib/yarjuf` which contains all internal and external dependencies

RSpec is now pessimistically versioned between `2.12` and `< 3.0` so we can make the next version of Yarjuf  (`1.0.6`?) the last that supports RSpec 2.

I've left updating the version to you or I can amend this PR before I refactor Yarjuf for RSpec 3 in a separate PR.

Ben