Switch from net/http to HTTParty for requests.

- Include HTTParty as dependency with Bundler.
- Require URI and HTTParty for entire gem.
- Update get method to use HTTParty methods instead of net/http.
- Remove unused methods and corresponding tests.
- Add method to include Access Token in query string hash with test.
Gemfile was improperly defined by using require.
Fix issue in Activity Data endpoint.

- Must include the required activityId parameter.
- Append the activity_id value instead of the required params.
Switch to rspec for testing.

Use rspec for testing instead of test/unit for more
flexibility and ability to use existing gems to help
with different kinds of testing.

Include the vcr and webmock gems with rspec to start
using mocks and fixtures to test the outbound API
calls to Nike without needing to always have a valid
internet connection. This commit only tests the
list_activities endpoint.

- Add vcr and webmock dependencies to Gemfile for bundler.
- Include configuration of vcr in spec_helper.
- Add fixtures for quick testing of endpoints.
- Begin testing of endpoints using vcr and webmock.
- Setup task in Rakefile for running rspec tests.
- Remove test/unit tests and Rakefile task.
- Translate existing tests into rspec.
- Include rspec as dependency with Bundler.
- Create spec_helper to include the NikePlusGem root file.