Add email validation for CCLA invitations

Any invalid email that gets input when sending CCLA invitations will not
be persisted and an email will not be sent, the invalid email addresses
are reported to the user and the valid ones are persisted and sent.
Support warning flash message

A warning flash message may now be displayed and the logic to choose the
appropriate class is now in the ApplicationHelper and tested.
Merge pull request #503 from opscode/invite-email-validation

Invite email validation