Additional changes related to removing US2ConditionEmpty and US2ValidatorEmpty

Updated a few comments as well as the main header file to use the new
condition/validator.
Merge pull request #22 from bachand/master

Updating a few comments as well as a header file to user US2ConditionPresent/US2ValidatorPresent
Updating podspec for version 1.1.0
Merge branch 'master' of github.com:ustwo/US2FormValidator
Merge branch 'master' of github.com:ustwo/US2FormValidator
Merge branch 'development'
1.1.1 - Fixes for #23 & #24
Merge branch 'master' of github.com:ustwo/US2FormValidator
Updating README