duration support for months

[baywet]

Is your feature request related to a problem? Please describe.
I'd like the ISO 8601 duration implementation to support months as well so it provides full support of the spec

Describe the solution you'd like

1. An additional field for Months on the duration struct
2. No more error to be returned if a months value is passed in the parsing function, and the field to be assigned
3. The String function to print that value when provided
4. The ToDuration function to include that value (what would be the conversion though?)
5. A validation rule to be implemented when week is used in combination with other units (prevented by the spec http://dotat.at/tmp/ISO_8601-2004_E.pdf)

Describe alternatives you've considered
N/A.

Additional context
Related #11