Allow users to specify formulas for custom numeric fields. Merged

We estimate our features (and sometimes requirements) at a functional level. This is then summed up to the overall time estimate, but we track the allocations for each group. Therefore, we need to be able to specify a feature estimate as:

Feature: Add a widget to a dashboard

BA Estimate - 5 days

QA Estimate - 5 days

Development Estimate - 10 days

Total Estimate - 20 days / 160 hrs.

This allows us to better judge the resource allocations within a product and releases as well as across product lines. This is pretty critical to us because we need to keep track of this information, so we would have to use Microsoft Excel to do this along side with Aha, just when we're trying to see if we can move to an all online platform. 

Other types of formula fields are applicable as well for dates and timing, but this is the most critical.