ProcessHelper.tryFindFileOnPath

Redoing #490 to use existing `tryFindFile` method. This is an attempt to help [port scripts that shell out to the command line](http://stackoverflow.com/q/24543048/906).

Any suggestions on how to test this? 

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Searches the current directory and the directories in the PATH
  environment variable for the given file.

Aims to replicate executing a command directly from the command line, where
  both the current directory and PATH are normally searched.