Add gitignore for the third party jgpx stuff

Hey Todd, 
Starting to work on this.  I'll take your start for setting up different devices and get my USB garmin stuff working.  

First off though, I have a couple pull requests about some simple stuff.  This is simply a convenience to add the jgpx stuff to a git ignore.

I'm new to the "using git to collaborate across different repos", so let me know if there is another way I should go about sending pull requests to you.
-M