Settings: Cache whether settings table exists

Before, ActiveRecord executed a SQL query to check whether the table exists on each settings access.

This has a pretty high impact on controller actions accessing settings a lot. The following results show measurements for WorkPackage#index on my MacBook.

40-100 Work Packages shown (not sure any more), Core only, fix enabled around 11:41:

![screen shot 2013-11-08 at 11 50 58](https://f.cloud.github.com/assets/188986/1500060/118fe80a-486c-11e3-994e-bfb12d3a08a3.png)

500 Work Packages shown, openproject.org configuraiton, fix enabled around 12:32:

![screen shot 2013-11-08 at 12 38 32](https://f.cloud.github.com/assets/188986/1500065/36427140-486c-11e3-8a6e-beeff297ffaa.png)

The queries executed for checking whether the settings tables exist are included in the 'SQL - SHOW' time.