Added type to options object

The option object now supports a type key/value, the application will throw an error when an unsupported type is requested.

See qrencode -help for supported types.
Removed version bump as per etiquette
encoder now accepts null argument

As per https://github.com/freewil suggestion if no type option is passed the -t flag and parameter will not be passed to qrencode process
toString

Corrected broken toString() method.
Removed dependency on case for type argument

Tests for changes to the default object, tests that a toUpperCase string exists in the allowed types.
Merge pull request #7 from lanoid/master

Added type to options object