Add Help screen

[barbeau]

We should information about how to use the app, as well as multiple attributions here, including to Mike Lockwood for original development of app, as well as attribution for Google Maps API v2:

If you use the Google Maps Android API in your application, you must include the Google Play Services attribution text as part of a "Legal Notices" section in your application. Including legal notices as an independent menu item, or as part of an "About" menu item, is recommended.

The attribution text is available by making a call to GooglePlayServicesUtil.getOpenSourceSoftwareLicenseInfo

http://developer.android.com/reference/com/google/android/gms/common/GooglePlayServicesUtil.html#getOpenSourceSoftwareLicenseInfo(android.content.Context)

[barbeau]

Should also include credit for ShowcaseView used for tutorial in GPSTest, implemented in #21:
https://github.com/Espiandev/ShowcaseView

[barbeau]

Also add ActionBarSherlock.

[barbeau]

Also add android-switch-backport:
https://github.com/BoD/android-switch-backport

[barbeau]

Moving Google Play attribution to #73.