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