Implemented Multi-Currency Support (or at least the start of it)

Signed-off-by: David Thorpe <davzie@davzie.com>
Fixed some typos that returned the wrong data, fixed some docblocks
Ensured Currency Symbol contains trailing blank line as per Laravel style guides
Cleaning up a lot of code.
Adding to interface.
Work on how currencies are formatted.
Move formatting to billable trait.
Add to contract.
Set the currency locale from billable instance.