Allow passing of flags, checks, passing of custom xml files

[JeffMatson]

Added functionality to include the passing of the allow-root flag.
Local XML file loading.
Correct XML location since repo changed hands.
Checks for cURL available.

Yes, my commit messages suck. It's late. Need sleep.

[tnorthcutt]

@JeffMatson this looks good to me; tests ok. I'm not 100% clear on what --allow-root does – could you open another PR to add some documentation to the readme? Wouldn't hurt to add info about using a local xml file also, even though it is fairly self-explanatory.

[JeffMatson]

@tnorthcutt --allow-root passes the --allow-root flag to WPCLI. Otherwise, WPCLI will error when running as root. I'll update the readme soon on it.