If PyCrypto is not installed make the error more obvious.

A user was getting an error that looked like a problem with pure-python-otr
but the problem was that PyCrypto wasn't installed.
Merge pull request #24 from mmb/pycrypto_missing

If PyCrypto is not installed make the error more obvious.
merge changes from gajim repository
fix license header
gotr: fix and add comments to UI gettext strings
complete PKG-INFO
merge in from hg
don't touch plaintext messages in receiveMessage(), spit them out via NotOTRMessage
gajim-otr: handle html messages
gotr: fix line break handling (issue #25)
handle setuptools magic (closes #26)
1.0.0beta6
support arbitrary otr protocol version codes in query messages, add OTRv3 tag
extra symmetric key support (closes #29), fix getDefaultQueryMessage() in py3k
fix mpi padding before several verifications (fixes occasionally failing AKE or SMP)
update convertkey to current version (closes #27)
Merge branch 'master' of github.com:afflux/pure-python-otr
fix links in README
Some incomplete instance tag support.
(Not sure what's missing, I wrote that code a few weeks ago)
Wrong branch. Revert "Some incomplete instance tag support."

This reverts commit 17c0df141f108f5ec4f3b914535151a5b620318e.