Invoice price splitter needed

This patch clones the Order PriceSplitter class and interface, and modifies it to support InvoiceInterface and InvoiceItemInterface.

It also replicates the PriceSplitterTest class to provide test coverage.

(It also adds a .gitignore file, also copied from commerce core.)

Try that one more time with the complete diff on the patch. :)

There is actually an issue in the order price splitter when splitting a negative fixed amount, that was carried over into this implementation.

This patch addresses that issue, along with adding an additional test case to test that scenario.

I'll also be submitting a similar patch in commerce core to keep them in sync.