Fix yas-snippet-dirs to append rspec-snippet-dir

The first directory in yas-snippets-dirs is used as the default location
to save a new snippet to. The default documentation for running
rspec-install-snippets will add rspec-snippets-dir to the list very
late. In order to be respectful to the users defaults we should append
rspec-snippet-dir to the list instead of placing it at the beginning.
Merge pull request #74 from dgtized/install-snippets-should-append

Fix yas-snippet-dirs to append rspec-snippet-dir
Use prefix with rspec-toggle-spec-and-target

With prefix argument, rspec-toggle-spec-and-target will do
find-file-other-window
Fixed doc string
Split rspec-find-spec-or-target-other-window off as a new command

* Add a dedicated keybinding
* Rewrite docstrings
Quote file paths

Fixes #77

+ Small change that allows `rspec-dired-verify-single' to use Rake.
Fix a typo and add an article