using resolve-bin to find the location of executables

I removed the `cmd.findExecutable()` in favour of https://github.com/thlorenz/resolve-bin - it checks the package.json and reads the `bin` property to look up executables.

Previous implementation relied on npm's `.bin` folder. It was causing issues, since on Win executable has `.cmd` extension, which makes it unusable if you commit your `node_modules` folder into source control (for the app). Previous implementation also relied on the binary actually being executable - now it just calls them via `node` (read from `process.execPath`).

We've been using this approach for over a week on Mac and Win - seemed OK, although more feedback would be nice. Works with both - local and global modules (i.e. npm's standard rules apply).