Add ability to retrieve user's activities.

- Include list_activities file in main nikeplusgem file.
- Update get method to accept query string params.
- Create list_activities file that adds specific method to Client class for retrieving data.
Add new file to handle extension methods for base classes.

- Include new 'only' method on Hash object to return a hash object with only the keys defined (whitelist).
- Add appropriate test for the new 'only' extension method on the Hash object.
Create helpers class to handle common tasks.

- Create initial method format_date to take different types and format into specific format.
- Add test for new format_date method.
Develop rest of the Nike+ API endpoints.

- Add the missing endpoints that haven't been defined.
- Remove the puts call with the uri under get.