add maxmem config option in compile tasks

provide quick and easy config option support to configure the heapsize of the forked ant task. it's equivalent to adding `-Xmx128m` to the arguments passed to the aspect compiler.

i set a default of 124m, because my machine appears to have a default max of 124m for a normal vm.

```bash
java -XX:+PrintFlagsFinal -version 2> /dev/null | \
    grep MaxHeapSize | awk '{print $4}' | xargs -I{} echo {}/1024/1024 | bc
```