Update jQuery

This pull request updates to jQuery 1.9.1 and adds the migration plugin to find compatibility issues. As soon as jQuery 2.0 is release we should be able to drop that in, because the API is the same and because we don't support old IE in the backend.

This closes #1610.

### Please Note

- This should only be pulled **after** the release of Symphony 2.3.2 final.
- The migrate plugin need to be removed again **before** the release of Symphony 2.4 final.