FireGento_Logger_Model_Mail: Character-Encoding of Mail Body

The default content-type of outgoing logging emails is:

    Content-Type: text/plain; charset=iso-8859-1

As not all sites use iso-8859-1 as charset, an option has been added
to the mail-logger to allow setting your own charset parameter value.

- Added charset setting to mailconfig
- Instantiate Zend_Mail with charset

Ref: https://github.com/firegento/firegento-logger/issues/35