create an interrupt button for R, send the commands to the server, record the processes that we would interract with.
store the unix PID of the R session so we can send it a signal to interrupt it
identify interruption vs other errors and report it so it looks like python
merge from master
suppress weird extra stderr line from R master process
disable on windows
reduce duplicated code, use bkHelper.httpPost
Merge pull request #541 from twosigma/spot/44

Fix Issue #44 Make R evaluation interruptible