Add ability to load partial templates from static

So that we can have shared templates and styles all served from static.

- Add a resolver which will load templates over the network from static,
  it only responds to templates which are prefixed with
  'govuk_component'
- Add helper class so you can include the shared templates with one
  line, this abstracts away the resovler so that the implementation
  could be changed in the future
- Add some basic tests to ensure that the templates are requested from a
  known location. If this code is refactored in the future we need to
  make sure the template location remains the same.
Allow intergration tests to test components

So a network request isn't created in tests respond with a predefined
html block.

Create a test helper which generates a components selector for use in
a test framework.

The testing of the component and how it responds with different
arguments can be tested where the template exists. There is also a
further bit of work to create a intergration suit which will confirm
it runs correctly in preview/production.
Merge pull request #76 from alphagov/element-partials

Add ability to load partial templates from static