Pay custom PoPR by link with multihash

[fazo96]

The /pay URL works just as before

However a new /pay/:multihash URL is now available, the difference is that instead of calling the Marketplace to generate a PoPR, the wallet will load the PoPR from IPFS at the given multihash and use that for the payment and review.

This allows third party applications to link to the wallet and request a specific PoPR to be paid.

How to test

• first open /pay. In the console or by inspecting the HTTP response you can get the multihash of the PoPR generated by the Marketplace. Or just use QmfSsqHV3CwAkfj72uE5Rpw5ghA4M2NhZYeafEJnGRkJuK
• open that PoPR with /pay/multihash and see that it does not make any HTTP requests.

[kulpreet]

Also, should we fix the travis config file to run the tests? Even if there aren't any. Would be nice to seen green.

[fazo96]

I think there are tests but they are from before the wallet-design refactor so they are useless now anyway. I will clean up a bit