Add dom-crawler development dependency

Adds composer to satisfy development dependencies. The dom-crawler component
will be used for unit tests involving the handlebars templates.

I'm not 100% thrilled with the conflict between our existing vendor directory
containing lightncandy and pimple, and composer using vendor. At the moment I
added relevant gitignore lines and this will work, but its not future proof
for expanded development dependencies.

These dependencies are not currently installed by the WMF CI system, we will
need to check with hashar about the best method to have these available.  For
now tests will need to check that the classe exist and use markTestSkipped when
not available.

Change-Id: Iaddc36f40338ff8603c4d5b3b4c550d0933dde41