Check to see if message is already encoded in utf8

Adding requirement to if statement to check if message matches UTF-8 encoding via regex. This will prevent encoding errors when converting UTF-8 to UTF-8.
Switch to mb_convert_encoding

Changing from iconv to mb_convert_encoding. $charset = 'UTF-8//TRANSLIT' is no longer needed. 'UTF-8' set as default and user may change value to whatever they wish :)
Merge pull request #1 from alexkavon/patch-2

Patch 2