Add ability to control start and end date for Agile Gadget charts...

The start and end date used in drawing the Agile Gadget graphs is derived roughly as follows:

1. Before preparing the chart, issues are filtered:
   • Issues that aren't in a GreenHopper-enabled are excluded
   • Issues that don't have at least one unreleased fix version selected are excluded
2. The list of versions is created based on the fix version associated with issues included in the filtered list.
3. The earliest start date is selected from the list of versions.
4. The latest end date is selected from the list of versions.

The end date for each version is determined by looking at (in order):

1. The end date configured in the planning board when viewing the version
2. The release date of the version
3. Today's date

This can lead to unexpected behaviors, such as when viewing a filter that uses "child" versions. Since "child" issues are also associated with the "master" version, the end date of the "master" version is included in the calculation. This makes it difficult to construct cross-project graphs that are targeted to specific "child" versions.

Adding the ability to set an end date manually (or to peg the end date to a specific version) would reduce the confusion.