Synchronous compile

I added an option to call node-sass synchronously, as we have a usecase where compiling the scss sources takes up all of the memory and cpu power of our development machines (large and many scss files).
The high load on the system resources completely killed the performance gains we made by switching from ruby sass to libsass. By compiling the files in sequence we went from a buildtime of over 2 minutes, to about 15 seconds again (and from 6Gb of memory and 97%+ cpu used by the node process, down to peaks of ±800Mb of memory and 25% of cpu (not a true benchmark of course)).