Improvement to allow extension of sortableOptions

Allow that if the sortableOptions is already available in scope that it gets used so i can tweak the D&D options more
Reverting double extend thingy
Adding unittest and reading sortableOptions from dashboardOptions
Updating README documenting sortableOption
Moving dashboardOptions to controller so that there is no duplicate code
Merge pull request #59 from sjoerdmulder/patch-1

Improvement to allow extension of sortableOptions