Integrate "inline images" patch

Original code: https://code.launchpad.net/~openerp-dev/openobject-addons/7.0-inline-images-in-mail/+merge/206433

Requires a patch against lxml to allow unfiltered "data:" value in the
<img src> attribute:

See https://github.com/jbq/lxml/commit/e55e8f09247d8477d87c36597acb54fd9dea5a5e