Update for XCode 6 beta 4 compatibility.

This commit adds public accessors and updates to the latest Quick and
Nimble releases.

Fixes #1.
Add Rakefile for running tests.
Move all types into a 1 file for easier install
Make initial parse class func, added docs, etc.

New usage for parsing is calling SWXMLHash.parse directly. Added
documentation based on http://nshipster.com/swift-documentation/... this
will likely change based on latter XCode releases.

Also, tests run successfully again (see 'rake test') with latest Quick
and Nimble pulls.
Improve docs.
Ensure that .all is accurate at all times.

There was a bug where .all only returned an array if there were multiple
elements found at the location specified. The problem is that XML could
be variable, so it might only return one item in one circumstance. Code
should be able to treat it as an array regardless.
Ensure indexing supports single elements.