Support for ignoring files when setting up watchers

Ignoring files is now possible by specifying a list of patterns for the `ignore` key of options.

The 'ignore' key allows a string or an array as value. If an array is used, both strings and
regex patterns are fair game. Resolves #11.



```javascript
var bundler = watchify({
  entries: './src/index.js',
  ignore: '/node_modules/'
});



```javascript
var bundler = watchify({
  entries: './src/index.js',
  ignore: [
    '/node_modules/',
    /\/vendor\/jquery-(ui|mobile).js/
  ]
});