work around new configobj behaviour

regarding utf8 chars in the config being read as str, not unicode
in python v2.7. This pathc introduces an additional
`alot.helper.string_decode` around the translated tagname read from the
config, and fixes issue #693. In the long run, we expect configobj
to be fully backweards compatible.
Merge branch '0.3.5-fix-workaround-new-configobj-693'
forward attached files in inline forwarding mode
properly wrap a forwarded message in message/rfc822 part
Move email_as_string function from crypto to helper module

The email_as_string function, and the related RFC3156_canonicalize
function, are now used by the ForwardCommand and are not specific
anymore to the crypto routine. So we move them to the global helper
module.

fix an import removal mistake while moving email_as_string function: StringIO was not only used by email_as_string
Merge branch '0.3.5-fix-forwards-682'
added check to bindings commands; added settings.get_keybindings(self,mode)
Merge branch '0.3.5-fix-bindings-help-691'
Add some basic Emacs keybindings in CompleteEdit
Add documentation and more shortcuts
Use Ctrl-b as in Emacs, not as in Vim
Document prompt key bindings
Merge branch '0.3.5-feature-promptmovements-706'