add mailto helpers

parse_mailto parses mailto strings into headers,body and
mailto_to_envelope constructs an alot.db.envelope.Envelope
for a give mailto string
interpret `compose mailto:foo@bar` commands
ensure that Envelope.body is unicode string
always return new Envelope in helper.mailto_to_envelope
non-mailto arguments to `compose` are recipients
interpret mailto and other parms to initial 'compose' separately
Merge branch '0.3.5-feature-mailto-666' into testing

Conflicts:
	alot/helper.py
	alot/init.py