Update Receipt class to deal with money strings.

Since the Taxable role interface changed, and this class
depends upon it, it has been updated to reflect this.
Update Parser module to pass money strings.

Taxable role interface changed, and now it expects a
string to represent a :unit_price, so the parser removed
the step of casting it to a BigDecimal.
Rewrite specs for LineItem::Parser

It now expects strings for :unit_price. In addition, the code
was simplified, using the #include expectation instead of #eql.
Add spec to line_item_parser spec

Spec is to test that the parsing brings back accountable
fields in the hash.
Update Parser#parse to delegate parsing to LineItem::Base

This is done to include accountable data in the returned hash.