Added account creation API.

Created new API action "createaccount" that
allows access to account creation. Takes
username, password, email, realname, token,
and optionally mailpassword and reason.

Errors are given in an errors array. Note
there is no way to natively handle CAPTCHAs
as there is no uniform implementation of
presenting CAPTCHA links to user. Right now
the best an extension can do is return an
error in AbortNewAccount, which will then
be displayed as an error in the Api result.

Change-Id: Ibdb1e50d434fb857683e1e4ff5a4a5a91c6b7c3a