Use environment variables

[basham]

Will need to use different URLs for APIs, for different environments. Dev, UNT, Stage, Prod. Figure out the best way to use different variable values for different contexts.

[basham]

http://stackoverflow.com/questions/28099041/gulp-build-with-browserify-environment-variable

[basham]

https://github.com/jas/gulp-preprocess
http://jbavari.github.io/blog/2014/08/23/managing-environment-variables-for-your-ionic-application/

[basham]

http://geekindulgence.com/environment-variables-in-angularjs-and-ionic/

[basham]

Maybe this is largely solved by #50. Would still be nice to investigate if variables could be set during the build process, instead of in HTML, though.