Setting hash fixed

The latest commit fixes three issues I was having. 

1) When trying to add a task from the Sprint page, it would throw a javascript error saying that a tracker wasn't found.
2) When trying to list tasks on the Task page, nothing would show up.
3) When in the settings menu for AgileDwarf, the settings wouldn't stick after setting them.

I traced this down to the fact that the settings hash as stored in the Setting model had strings for the keys rather than symbols. When I changed all the keys from symbols to strings, the problems all went away.

My Environment:

Environment:
  Redmine version                          2.2.3.stable
  Ruby version                             1.9.2 (i686-linux)
  Rails version                            3.2.12
  Environment                              production
  Database adapter                         Mysql2
Redmine plugins:
  AgileDwarf                               0.0.3