simplifying challenge storing
adding slug parameter to selectchallenge
wrapping challenge dialog

in fadeout complete callback for more elegant show / hide
more function wrapping

for more elegant fade in / out
done dialog must fade out here
simplifying userpickchallenge and passing slug parameter
Merge branch 'develop' of github.com:osmlab/maproulette into develop

* 'develop' of github.com:osmlab/maproulette:
Merge pull request #117 from mvexel/develop

improvements to ui, mainly challenge selection and (dis)appearing of elements