app: added CName field to App

Related to #293.
app: added SetCName method to App

Related to #293.
app: better wording in SetCName docs

Related to #293.
app: use "app" as the name of the target variable in all App's methods
api: added setCName handler

This handler will set the cname of the app.

Related to #293.
app: validate cname

Related to #293.
api: provide proper error code when cname is invalid

Related to #293.
api: map url to setCName

Closes #293.
api: receiving a json with app information in setCName

This url can be used later to update an app.

Related to #293 and #226 (quite).