Unbind the prefix binding when rspec-mode is turned off
Tweak keybindings and minor modes

* Fixed #25: since the hook for rspec-mode-verifiable-keymap went after the one
  for rspec-mode-keymap, the former override the latter. That's why
  `rspec-verify-single` keybinding didn't work until you called `rspec-mode`
  again.
* Clean up after rspec-mode when disabling. The situation with imenu vars is
  still not ideal (what if another mode changes them?), but better than before.
* Create rspec-verifiable-mode, so that the user can similarly turn the keymap
  binding off. Not too useful, but still a nice thing to have.
* No need to wrap `add-hook` calls in `eval-after-load`. `add-hook` doesn't care
  if the hook variable has already been defined or not.
* Clean up extra empty lines; add a missing one.
Use fixed keymaps for both minor modes

`local-set-key` docstring says: "The binding goes in the current buffer's local
map, which in most cases is shared with all other buffers in the same major
mode." It breaks our keymaps one way or the other as we open new Ruby files.

The only drawback I see here is that rspec-key-command-prefix can't be changed
once the mode file is loaded. If that's a desirable feature, we can add a
defcustom setter.
Use compilation-scroll-output, remove rspec-end-of-buffer-target-window

The effect seems to be the same
Define rspec-compilation-mode, use it in rspec-compile

* Set only buffer-local values, should fix #41
* Get rid of (expand-file-name), it didn't work and broke the highlighter
* Include the whole (relative) file path in the first group
* Highlight "xx examples, xx failures, xx pending" in red or green
* Highlight Pending: and Failures: without relying in the default highlighting
Bumped version and added contributing secittion to readme.
Support spring