[ALOY-294] Ti.alloy compiler plugin should use PATH when possible for finding alloy/node

Right now the node and alloy paths are hard-coded. We need to take advantage of the PATH to find them. This will work relatively easily on command line, but we need to resolve ALOY-85 for this to work with TiStudio. That fix for TiStudio requires the latest 3.0 build, specifically 3.0.0.201209181626 or greater. Additionally, Alloy should guess at a couple common paths if it can't find node/alloy on the PATH. Finally, we should define 2 environment variables to allow devs to point to custom paths not covered by the aforementioned searches. ALLOY_PATH - path to the alloy binary ALLOY_NODE_PATH - path to the node binary that alloy should be using