FreeSWITCH-FS-7447

Added new options to nibble bill for minimum charges and rounding #FS-7560

These options are:

1) nibble_rounding: The number of decimal places to round the cost of the call to
2) nibble_minimum: The minimum charge to apply to a call

Rounding is applied before the minimum charging, and these steps only occur at call end to make sure they're accurate.

Checks for minimum_charge before call starts

"Billing done" flag to the data struct, and used that to exit early in the event processing during hangup has already occured.