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 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.