Support custom charset

See #51, release log:

• 7d6fc3c by @rultor: Merge branch '__rultor'
• 3bab3aa by @valery1707: Fix Maven configuration in App...
• 22d8d56 by @valery1707: Use custom encoding in Stamps
• fe3a4c2 by @valery1707: Use custom encoding in Subject
• fd6fee0 by @valery1707: Use custom encoding in enclosu...

Released by Rultor 1.64, see build log