fix timing issue with model/property create

- if a user clicks the new property button directly
after blurring the model name input it will create a
race condition where the mode must exist before a
property can be created.
- there is a property that is set during the model create
async process indicating the model is being created
so we are now checking that property and looping until
it has been reset to creating=false
Merge pull request #806 from strongloop/fix/802-model-property-error

fix timing issue with model/property create