Allow to use a text logo when logo_url setting is set to an empty value

Added a conditional that now tests if the logo_url is empty. In case it is, the logoHTML function now returns a header tag with the site_title as its value, instead of returning an image tag that would only show the alt text in the browser.

Situation: SiteSetting.logo_url = ""

Before changes:

![screen shot 2013-06-20 at 1 28 53 pm](https://f.cloud.github.com/assets/2272207/682409/9b57dee0-d9c6-11e2-93c8-dfc85760c833.png)

After changes:

![screen shot 2013-06-20 at 1 15 58 pm](https://f.cloud.github.com/assets/2272207/682317/db1c9478-d9c4-11e2-8851-4c5908105c7a.png)