Adds support for json-based widgets/mashups

[dmunozv04]

Proposed changes

Implements the necessary changes so json-based widgets/mashups can be loaded/created in wirecloud.
Creates mashups using a config.json properties file. Adds a jsonschema validator

Types of changes

What types of changes does your code introduce to the project: Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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• I have read the CONTRIBUTING doc
• I have signed the CLA
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

It would be interesting to change the config file to something unique instead of config.json so the JSON Schema can be sent to Schema Store to allow verifying the config file inside IDEs. Reopening this PR since the last one had some issues with files from another branch

[coveralls]

coverage: 88.217% (+0.007%) from 88.21%
when pulling 9a404e8 on dmunozv04:json-support
into 2a25766 on Wirecloud:develop.