Persistent cache

Code for persistent cache. It should be free from syntax errors but build fails with the following output:

```
[INFO] Reactor Summary:
[INFO] 
[INFO] mapsforge .......................................... SUCCESS [  2.745 s]
[INFO] mapsforge-core ..................................... SUCCESS [  9.434 s]
[INFO] mapsforge-map-reader ............................... SUCCESS [  4.117 s]
[INFO] mapsforge-map-awt .................................. SUCCESS [  3.399 s]
[INFO] mapsforge-map ...................................... SUCCESS [ 12.661 s]
[INFO] mapsforge-map-android .............................. FAILURE [  1.152 s]
[INFO] mapsforge-map-writer ............................... SKIPPED
[INFO] Samples ............................................ SKIPPED
[INFO] SwingMapViewer ..................................... SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 35.733 s
[INFO] Finished at: 2014-12-14T17:26:01+01:00
[INFO] Final Memory: 32M/306M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal on project mapsforge-map-android: Could not resolve dependencies for project org.mapsforge:mapsforge-map-android:jar:dev-SNAPSHOT: Could not find artifact android:android:jar:5.0.1_r2 in central (http://repo.maven.apache.org/maven2) -> [Help 1]
```