decrypt support of PGP-Inline messages

Added support for decrypting PGP-inline messages as discussed in [pull request #2.](https://github.com/jkraemer/mail-gpg/pull/2#issuecomment-25773783)

The code was tested by generating PGP-inline messages with the Mozilla Enigmail plugin, and afterwards by generating test cases mimicking these messages.

As I needed to reuse the GPGME decrypt method from decrypted_part.rb I moved this method into a new gpgme_helper.rb file. To keep the library consistent I also moved the GPGME encrypt and sign methods into this file as well.

Please don't hesitate in criticizing the code in case you think I missed something, or hacked to much.

Best Regards
Morten