Consolidate version string into build.gradle, fixes #86

Versioning is now completely controlled in build.gradle
BuildConfig.VersionName and BuildConfig.VersionCode
Moved a number of strings to resources, fixing #93

I may have missed one or two, but we will find them when
doing a translation.
Refactor literal uses of URL into one place
Merge pull request #109 from rfay/20141102_consolidate_strings

Consolidate strings and resources where possible