Add validation to ensure that web.spa.enabled isn't set without a web.spa.view

[typerandom]

Adds validation so that we ensure that web.spa.enabled isn't set without a web.spa.view.

Discussion

What do you think about the error message? Is it descriptive enough?

How to verify

1. Checkout this branch.
2. Run $ npm link.
3. Checkout and link the stormpath and express-stormpath stormpath modules with stormpath-config.
4. Clone express-stormpath-sample-project.
5. Use npm link and npm link express-stormpath to link the library to the example project.
6. Open up app.js and set web.spa.enabled to true.
7. Start the server.
8. Verify that the SPA mode is enabled but web.spa.view isn't set. This needs to be the absolute path to the page that you want to serve. console error error is shown.
9. Open up app.js and set web.spa.view to abc.
10. Start the server.
11. Verify that that no console error is being shown.
12. Open up app.js and set web.spa.enabled to false and web.spa.view to null.
13. Start the server.
14. Verify that that no console error is being shown.

Fixes #36.

[robertjd]

Verified, merging and will make a patch release so that we can use this immediately.