Support for incremental ANT builds for Android

[teobugslayer]

Issue

Android build are unnecessary long: 12s or more.

tns build works by always overwriting files in the app directory and then spawning ant to do the actual building. This way ant always sees new files and cannot trigger an incremental build.

Improvement strategies

We can improve the situation in two ways:

• tns prepare become "smarter" and only copies files newer than existing apk. This makes tns dangerously close to a build system which we should avoid.
• instead of copying files into app, symlink them. This may have issues on Windows (see "tns platform add android --path TNSApp --symlink" does not work on Windows #282 ). In this way, prepare will just create symlinks for new files.

This issue supersedes NativeScript/android#60 .

[dtopuzov]

The issue with incremental builds will be solved once #687 is implemented.