Automatic deprecation and code style fixes

[FIX] #12 utilizing full path component
[ADD] config option to use global 'mailfrom'. solves #11
[FIX] #10 properly clean up recipients string before explode

Handle missing files

Let mailer handle placeholder replacements

Replace unused setting with option to disallow emails to arbitrary
addresses

Include subject in mail templates

Autocomplete for recipient field

Recognize groups as valid email recipients

Load message templates in recommendation form

Admin interface for managing snippets

Cosmetics

Igor compatibility and replced old Doku_Form with modern Form object