Add heading

This change adds heading and course accuracy data from the ```UBX-NAV-VELNED``` message to the log file. At high speeds, the ```heading``` field agrees closely with heading calculated from ```velN``` and ```velE```. However, at low speeds the two differ somewhat. For this reason, even though it may seem to be redundant information, the inclusion of the ```heading``` field seems warranted. The ```cAcc``` field provides [circular error probable](http://en.wikipedia.org/wiki/Circular_error_probable) estimate of heading accuracy.