Small improvements to gradle config

This fixes a bug in the calculation of `versionCode` and `versionName`.
It also introduces a new method of calculating these values.

Example: 
`versionCode` = 9700 for `versionName` 0.9.7
`versionCode` = 12300 for `versionName` 1.2.3

Previous output from master, using `$ aapt dump badging build/apk/Catroid-debug-unaligned.apk | grep -i version`
```
package: name='org.catrobat.catroid' versionCode='14' versionName='0.9.-1'
sdkVersion:'10'
targetSdkVersion:'18'
```
Output after this PR:
```
package: name='org.catrobat.catroid' versionCode='9700' versionName='0.9.7'
sdkVersion:'10'
targetSdkVersion:'19'
```

Also new:
BuildTools 19
targetSdkVersion 19 (KitKat)

@xorrizon, what do you think?