Add DxCompile Task

This task gathers the class files of a target and compiles them into a classes.dex.
Dex file is a Dalvik exeecutable file. Once the classes have been dexed, then the app
is ready to be packaged into an .apk.

This is the task itself, I will write the test class tomorrow. I wanted to give people
a chance to comment while I worked on them, in case  large changes are needed.
If you prefer to wait for the test class, that will hopefully be up tomorrow.