Added Costings to Billing

Added support for costings. Apply a base and burst rate per Mb for CDR or Quota.
Calculates anything up to 150% of CDR at base rate, then the rest at burst rate.

Changeable currency symbol via:
$config['billing']['currency_symbol'] = "£";

Also changed some of the billing calculations to reflect $config['billing']['base'] more.

I've only tested this internally, probably needs more rigorous testing before merging.

Hopefully someone else will find it useful!

Cheers,