Currency Upgrades

Hi Folks

Was using this to upgrade a reporting tool we have but found it a struggle to work with multiple currency values. Instead of handling outside I have upgraded the internals of the tool.

In this PR: 
- Default Report Currency Symbols
- Row Level Currency 
- Max / Min aggregates

Here's an outline of the changes. 
https://github.com/nburdy/pivot.js#notable-changes-in-this-fork