Add gettext:setup task

* Allows application developers to setup their domains
* Allows overiding default files to translate

Closes #107 and #109.
DRY the msgmerge and xgettext options
Add documentation for gettext:setup task
Read default options from Rails if defined
Add documentation explaning the msgmerge and xgettext options
Merge pull request #108 from skroutz/add_setup_task

Add gettext:setup task