ProcessHelper.findExecutableInPath

Attempts to locate an executable file normally run from the command line.
Will search the current directory, then the paths in the PATH environment variable.
**Caution: no automated tests. If this is something useful for FAKE please review carefully before merging. :)**

This is an attempt to deal [porting scripts that shell out to the command line](http://stackoverflow.com/q/24543048/906).

Here is an [example of using it](https://github.com/dtchepak/NSubstitute/blob/0f45567df3adde18124bb5ef8969824015b21f5e/build.fsx#L131) to shell out to `bundle exec jekyll docs`.