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/docs-package/mocks/mockPackage.js

@@ -0,0 +1,10 @@
+var Package = require('dgeni').Package;
+
+module.exports = function mockPackage() {
+
+  return new Package('mockPackage', [require('../')])
+
+  // provide a mock log service
+  .factory('log', function() { return require('dgeni/lib/mocks/log')(false); });
+
+};