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
Compromise between mb_convert_encoding and iconv
Second $charset should of been $charsetFlag
whitespace
picky travis, added else statement
Merge pull request #2 from alexkavon/patch-2

Patch 2
bug fixes
Updated message for $charsetFlag
bug fixes, message changes
variable and message fix
head conflicts fix
Merge pull request #66 from alexkavon/patch-2

Check to see if message is already encoded in utf8
Implement memory-efficient streaming save
Merge pull request #70 from skl/stream-save

Implement memory-efficient streaming save
Fix incorrect variable name
Merge pull request #71 from skl/stream-save

Fix incorrect variable name
Added a dependency
Give Message class constants to access the imap flags