Extend the API with 'observeVerificationRequests' and an additional parameter

[n9iels]

Description

This PR add the observeVerificationRequests method to the API. This makes the implementation of connectors that require additional verification before a claim can be verified possible. An example of such a connector is IPv8 connector

Another change in this PR is an additional parameter for the claim method. This parameter, named attester, makes it possible to express the did of the expected attester. This makes it possible to implement a connector that requires both parties to be known when creating a claim.

Backwards compatibility

No breaking changes are made in this PR

[n9iels]

Adding integration tests is possible since there is already a implementation for this in the discipl-core-ipv8 repo itself.

I do need to say that the integration tests currently are not stable. Sometimes they randomly fail and after a restart they pass without any problems. I will take a look at that issue first (https://github.com/discipl/discipl-core-ipv8/issues/15), after that I will implement it here.

[n9iels]

@pimotte I added some integration tests for the IPv8 connector. Once the npm package is live and installed Travis should be happy too :smile:

[sonarcloud[bot]]

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