Tool for pinning and unpinning IPFS objects

[cuongdo]

Checklist:

• [x] Code contains relevant tests for the problem you are solving
• [x] Code has been formatted with autopep8 --in-place --recursive --a --a .
• [x] Ensure all new and existing tests pass
• [x] Update any relevant READMEs and docs
• [x] Submit to the develop branch instead of master

Description:

This PR adds a new tool called ipfs_pinner.py for pinning / unpinning IPFS hashes depending on whether they have a corresponding Origin listing. This is meant to be run as a cron job. ipfs_pinner.py inherits its settings from bridge-server.

[cuongdo]

I incorporated @wanderingstan's suggestion of getting the IPFS hashes related to Origin listings directly from the blockchain. This works much better now, at least in my local test environment (with a local IPFS, blockchain, etc.).

[tyleryasaka]

It's been a while since I've run the bridge server and I'm running into issues getting it running now, so I'm not able to test this just yet. Code LGTM and I'm working on getting my environment set up properly. Will hopefully be able to test soon.