add callable location

In flask==0.10 added get_json request argument which will remove json in future: http://flask.pocoo.org/docs/api/#flask.Request.json.
It also can be actual for another location in 0.10.
Perhaps my test for check callable with get_json will fail for flask==0.9 and less.
Merge pull request #107 from tbicr/source-from-callable

add callable location