Issue 8 simplify app dev

This commit implements the proposed work flow by @gka for PyBossa. Basically, it allows to pre-load tasks (improving the performance) as well as simplify the code.

The changes are:
 * pybossa.taskLoaded, pybossa.taskPresenter and pybossa.run methods.
 * pushState usage for automatically change the URL without leaving the page.
 * Unit tests for these new methods.