BuildServer: pre-dex the runtime libraries

I replaced the current dex launching method with a heavily modified version of the Android SDK's Dex ant task. Now, if you pass in a directory that dex files can be cached into, dexing time and memory usage goes down dramatically after the first launch.

On my crappy 6-year old laptop with 1GB of RAM, OpenJDK 32 (Client VM) running Ubuntu 12.04, and with a Pentium M processor 1.20GHz, I get:

Without pre-dexing, running ant PlayApp

```
BUILD SUCCESSFUL
Total time: 3 minutes 59 seconds

real	4m0.929s
user	1m49.631s
sys	0m6.796s
```

With pre-dexing (All dex files in cache except project classes):
```
BUILD SUCCESSFUL
Total time: 1 minute 2 seconds

real	1m4.228s
user	0m44.087s
sys	0m3.376s
```