[#164] Implement 'External Source' feature

* Read flag from XML file
* Display/Hide button in free-text questions
* Delegate question response through custom intent action
* Read the response and notify question
[#164] Refactor onActivityResult() function
[#164] Cache question ID
[#164]Merge branch 'develop' into issue/164-caddisfly

Conflicts:
	app/src/main/java/org/akvo/flow/activity/SurveyActivity.java
	app/src/main/res/values/strings.xml
Merge pull request #198 from akvo/issue/164-caddisfly

Code checked