Stop clearing main module's paths

[nickclaw]

Closes #34 and #56.

Without this fix, the following code fails with: Error: Cannot find module 'express'

var pmx = require('pmx');
pmx.init();

setTimeout(function() {
    require('express');
}, 1000);

Happy to add a test case if you need it, wasn't sure where to put it.

[Unitech]

interesting, could you put a test file for this here: https://github.com/keymetrics/pmx/tree/master/test

[nickclaw]

@Unitech done!

[Unitech]

awesome thanks