Add the email to the flash messages in Registration.handleStartSignUp

This is done so that the email address that the user shall check can
be displayed in the target page / template.
Add support for plain text, only html and multipart emails.

The templates plugin already allows to return customized content
for the emails. Though, the email content is already sent as content
type html.
This improves it to allow sending only text, only html or multipart
emails. The templates plugin now returns a tuple of Option[Txt] and
Option[Html], so that clients can choose which parts they want to
provide. As the mailer plugin handles null or empty content gracefully
we can reduce None simply to null or an empty string (the latter is chosen
because the mailer plugin also passes an empty string if a part shall
be omitted).
Update documentation for TemplatesPlugin text/html/multipart support.
- Added a makeTransient property for the authenticator cookie.
- Fixed: absolute timeout was not being picked up from the conf file
- clean securesocial values after login from the session
- renamed keys used in session
updated change log
update version in docs
updated docs
Merge pull request #162 from magro/plain-text-html-and-multipart-emails

Add support for plain text, html and multipart emails in TemplatesPlugin
Merge pull request #161 from magro/add-email-to-handleStartSignUp-flash

Add the email to the flash messages in Registration.handleStartSignUp