Add a "serve" command to "closure-util" binary

[elemoine]

This PR adds a "serve" command to the "closure-util" binary. This command starts a development server that can be used to serve code using goog.provide and goog.require.

This is how the serve command is used:

closure-util serve config.json

And this is what the JSON config file (config.json) looks like:

{
  "lib": [
    "path/to/my/js/files/**.js"
  ],
  "serve": {
    "port": 3001
  }
}

That configuration file can be the same file as the one used with the build command (with an additional "serve" section).

Requires #52. (now closed)

[elemoine]

Now rebased on the current master.

[elemoine]

I just updated the description of the issue. Please review.

[tschaub]

I'm sure you know, but the _privateStyle names are not really necessary. Only properties explicitly assigned to exports or module.exports are available outside this module.

[tschaub]

Looks great @elemoine. One minor comment about the _namingConvention, but that is a stylistic thing (and not yet enforced).

Please merge (and release).

[elemoine]

Looks great @elemoine. One minor comment about the _namingConvention, but that is a stylistic thing (and not yet enforced).

I'll remove the underscore before merging. Thanks for the review.