Merge pull request #87 from sambohler/master

Fixed Client->createInvoice, wrote tests to match
Updated construct to take the currency code as an optional argument
Added available currencies and updated the constructor and setCode to the currency class
Added phpmd to project
Updated documentation to some some conventions, updated docs to show the new usage of the currency class, also updated the testing documentation to show how to use phpmd
Small bug fix and made the configuration file have 100% code coverage
Code cleanup, removed unused variables
Updated dev dependencies to patch level
Removed the sin_key option since it will be going away soon

Removed Robo in composer.json since we do not use and updated travis config to allow failures in php 5.3 since we require 5.4 or greater
Deleted robo file

Large update with documentation updates, extra code functionality, and
bug fixes.
Merge pull request #88 from JoshuaEstes/SmallUpdatesThatMakeThingsFancy

Updated construct to take the currency code as an optional argument
Updated invoice so that full notifications can be enabled
Minor logic bug fix

isGenerated check causing extra generation.
Merge pull request #89 from JoshuaEstes/SmallUpdateToInvoiceClass

Updated invoice so that full notifications can be enabled
Fixes #75, documented failure return type