Make Fixed Membership Types Require the Duration Unit Be Expressed in Year(s)

As per email conversation with Dave:

A fixed, one-month-duration membership renewed on July 4th with a fixed period start day set to June 10th and rollover day set to July 1st gets renewed 'till July 10th, not for the period of July 10th-August 10th. This is because our current Fixed Period model isn't really set up for duration units other than YEARS - since the Fixed Period Start Day and Rollover Day are expressed as Month + Day.

We should add a form rule that rejects Fixed membership types with Duration Unit != 'year' (and note that on the "Period Type" field help).

We should also adjust Scenario 5 from our 'Test Plan for Membership Renewals' wiki page accordingly: http://wiki.civicrm.org/confluence//x/ry8