Add units selector to measure dialog. Add info text to dialog

This PR adds a unit selector and calculation information to the measure dialog to help to user control the output.

The values are updated live when the combo is changed and the info text explains the logic of the calculation.  With OTF enabled it will also tell you which ellipsoid is being used. 

![measure otf off _005](https://cloud.githubusercontent.com/assets/381660/5592099/ee5bf50c-91ff-11e4-88d2-c9d855ae0966.png)
![measure otf off _007](https://cloud.githubusercontent.com/assets/381660/5592098/ee48c6da-91ff-11e4-8d43-61ed07f288fd.png)
![measure otf off _008](https://cloud.githubusercontent.com/assets/381660/5592100/ef0d2a98-91ff-11e4-9fee-72eddaea994e.png)