Feature/490 json attributes as object

### Description
The rendering of ContextAttribute for JSON has now two possibilities:
1. As a vector (the old way)
2. As an object, with each attribute as an object inside

(1) is the default rendering method, while the second method is achieved adding the URL parameter

    attributesFormat=object

Fixes issue #490.
We should be careful with chaning style *outside* the PR: note that this PR is not going to release/iotplatfor-v1 (as it involves a feature addition) so most of the style fixing in this file is not going to end in release/iotplatfor-v1. We can even have merging conflicts in the future if a fix in the style changed part in release/iotplatform-v1 has to be also ported to develop branch...

NTC (just a "warning note")