added jvm arg element capability to flex unit ant task

[sonnyg]

I also updated the flex sdk to 4.15. My apologies if this causes any unintended issues.

On the other hand, it is possible to set the memory for the underlying Java task using jvm arg elements in the ant script.

Like so:

<flexunit ...>
   <jvmarg value="-Xmx${mxmlc.max.memory}" />
   ...
</flexunit>