Misc non-functional enhancements

1. Addressed miscellaneous outstanding (non-functional) review comments
from the previous refactoring and 3.7 update check ins:

a) better names for some APIDefinitions and added more comments
b) remove unneeded getCustomViews(DataViewType) method in UsersService
(getCustomViews(CustomViewApiType) should be used instead)

2. Changed implementation of the uploadDocument(JSONDocument) (in
DocumentsService)

Some Mambu Mobile users reported crashes with “out of memory” errors in
the wrapper while running Gson.toJson(document) with large attachments.
New custom implementation avoids unnecessary memory allocation in this
method. Tested with large files.

3. Minor implementation change in addAppKeyToJson() in
RequestExecutorImpl  (changed to appending JSON string instead of
inserting into it).
Updated comments
Merge pull request #34 from mdanilkis/master

Miscellaneous non-functional enhancements