Refactored InvoiceParams class with properties. Changed KeyValuePair to Dictionary and other minor changes.
Refactored invoice to use properties, renamed a method, fixed tests to work with changes
Merge pull request #3 from unChaz/feature/refactor

Feature/refactor
Updated copyright
Updated readme with improved documentation
Merge pull request #4 from ionux/master

Improved documentation & updated copyright information
merge with upstream