Add ability to map source paths to module names.

Using browserify with express allows me to require npm modules directly from the node_modules path, e.g. require("bean"). karma-commonjs unfortunately resolves "bean" to a relative path to the source file which of course doesn't work, as that would be something like "../../node_modules/bean/bean".

This patch adds a "commonjs" config with one property "map" that allows to map any source file to a different module name. Example karma.conf.js snippet:
```
    [...]
    files : [
      {pattern: 'node_modules/bean/bean.js',      watched: false},
      {pattern: 'node_modules/domready/ready.js', watched: false},
      'browser/lib/*.js',
      'browser/lib/**/*.js',
      'browser/test/*-test.js',
      'browser/test/**/*-test.js'
    ],
    preprocessors : {
      'node_modules/bean/bean.js'      : ['commonjs'],
      'node_modules/domready/ready.js' : ['commonjs'],
      'browser/**/*.js'                : ['commonjs']
    },
    commonjs : {
      map : {
        'node_modules/bean/bean.js'      : 'bean',
        'node_modules/domready/ready.js' : 'domready'
      }
    },
    [...]
```