Implement verifychain method and test

[GideonBature]

The JSON-RPC method verifychain returns a standard type (bool). We would like to test this in case a later version of Core changes this behaviour.

This PR adds a client function that errors if the return value is anything other than bool, along with an integration test that calls this function.

Ref: #116

[tcharding]

Except the docs thing everything here looks good - too easy!

[GideonBature]

Except the docs thing everything here looks good - too easy!

removed the unnecessary lines of the rust docs, leaving just a single line, as you suggested....

[tcharding]

While reviewing this I saw that we have the integration test for gettxoutproof in the wrong place (it shows up in the diff of this patch). Want to fix that?

FTR I'm as fallible as the next bloke, if you see mistakes please clean em up - its super appreciated. This repo is even more necessary to keep clean than most because its so big, boring, boilerplate. In general you can either do them as separate PRs, or if the PR you are working is going to include the mistake in the diff just put the fix as a separate patch at the front of the PR i.e., do it first in a separate patch (commit).

[tcharding]

ACK ae286f1

[tcharding]

Thanks man, you are killing it!

[GideonBature]

Thanks man, you are killing it!

Thank you.... I am learning from you...