REComposeViewController: use keyWindow instead of delegate.window

I'm working on a project, where delegate.window isn't set.
UIApplication.keyWindow should be more generic & work for everybody.

Thanks for cool pod, really useful & convenient!
Added option for placeholder text.

Added option to set placeholder text in the DEComposeTextView
Merge pull request #17 from rocket0423/master

Placeholder Text
Version bump (2.1)
Rename placeHolderLabel to placeholderLabel
Merge commit '11325d91c0b277bdd6622d7611e4e1d2fb6e1220'
Update readme
Version bump (2.1.1)
Retina screenshot
Update README.md
Update README.md
Update README.md
Call completion block when REComposeViewController has been dismissed
Merge pull request #18 from peterwarbo/master

This change will run the completion block after REComposeViewController is dismissed
Add completion check against nil value
Update podspec
Update readme
[Update] 使用新的UITextView