Be sure to do UI things on the main thread.
Update Foursquare in Readme.
Add Foursquare to pod spec.
Add contributors to readme.
Put contributors under thanks section.
Rename foursquare provider.
Use defined options key.
Be sure to do UI things on the main thread.
Add Dropbox web provider.
Merge pull request #21 from calebd/dropbox

Add Dropbox web provider.
Get ready for v0.3.1 tag.
Fix type mismatch warning.
Make a new account store helper.

- Add class category for accessing accounts of a given type.
- Move Facebook provider to new account access pattern.
Some cleanup of the Twitter provider before I refactor it.
Finish refactoring system providers to standard provider API.
Merge pull request #25 from calebd/system-refactor

Refactor system providers.
Use old Ruby hash syntax.
Add ability to pass scope to Instagram.
Better cancel error handling.
Remove SimpleAuthSystemProvider.