Fix #163 - Add setting to change distance units

This includes an automatic option (default) that attempts to guess metric/imperial based on current locale (currently, it assumes Imperial for USA locale country setting and Metric for all others).  Automatic selection can be manually overridden by the user with explicit metric/imperial options.

Here's what the new preference category looks like:

![oba_preferred_units_resized](https://cloud.githubusercontent.com/assets/928045/3881073/7d8f4f76-218a-11e4-9936-100e009f002b.png)

... and here's the popup with options:
![oba_preferred_units_options_resized](https://cloud.githubusercontent.com/assets/928045/3881100/af886b48-218a-11e4-99a0-617505d389b4.png)

@paulcwatts This new preference didn't seem to fit cleanly into existing categories, so I created a new category "Display" for this, for lack of better ideas.  If you have suggestions for names, please let me know and I'll change the text.