GMTString and ClearButton

I'am new to git and don't know how to revert readme.md to yours. Beside the localisation problem which is solved with the toGMTString, I added a third button to clear time value. Pass a string to label the button and it will displayed, sadly "clear" is not included in R.string. ReturnValue is -1 because the empty string is already used by cancel, backButton and click outside.

Todo's: 
- Implement clear functionality to dateDialog
- by default the dialog is ugly in Android 2.3 Theme, perhaps you can mention to config the theme in the manifest file.