add http API for cancelExecution

I won't call this a REST API since I'm pretty sure I know how
the RESTful crowd would do it, and I didn't do it that way.
Lame partial porting of build.js to new sbt API

This doesn't accomplish that much, because the compile/run/test
tabs are still directly using the older build.run/build.compile/build.test
objects.

It may be easier to drop build.js and reconstruct the plugins using
sbt.js directly ? not sure.