Bug 56590: Use the Parsoid HTTP API in round-trip testing

roundtrip-test.js now uses a (configurable) Parsoid HTTP API to obtain html
and wikitext for a page before comparing to the original wikitext.
The round-trip test client also has a new configuration option for the URL of
the Parsoid API.

If no Parsoid URL is passed, the new module apiServer.js starts one for us.

Moved to new repo from https://gerrit.wikimedia.org/r/#/c/99348/  .

Change-Id: Ibb9daa72d51d91c6192ac0d695e6ecc4e70005aa
Merge "Bug 56590: Use the Parsoid HTTP API in round-trip testing"