chore(doc-gen): put typescript stuff into its own package

This means that we can now run just the d.ts file generation by running:

```bash
gulp docs/typings
```

In addition the type definition generation was messing with the other docs tasks
so separating it also fixes problems there.

docs/public-docs-package/index.js

@@ -1,7 +1,7 @@
 var Package = require('dgeni').Package;
-var basePackage = require('../dgeni-package');
+var basePackage = require('../docs-package');
 
-module.exports = new Package('angular-public', [basePackage])
+module.exports = new Package('angular-v2-public-docs', [basePackage])
 
 .config(function(readTypeScriptModules) {
   readTypeScriptModules.sourceFiles = [