Add date units to periods

[bonjourmauko]

Depends on #1138

Thanks for contributing to OpenFisca! Remove this line, as well as any other in the following that don't fit your contribution :)

New features

• Introduce DateUnit to periods
  • Allows for transparent encapsulation of the atomic units of periods (year, month, ...).
  • This helps with testing, refactoring and exrtension of periods (weeks for example).

[coveralls]

Coverage increased (+0.7%) to 79.676% when pulling f51fc62 on add-date-units into 3eb157f on master.

[benjello]

Looks nice @maukoquiroga.

[bonjourmauko]

Superseded by #1138