Rinkeby bug fixes + native libav support

This release includes a few bug fixes to account for the migration to the Rinkeby network.

• Retry logic for when requests to external Ethereum nodes time out.
• Minor bug fixes to the p2p networking layer

Another important milestone included in this release is full native libav integration. With this upgrade, the livepeer node no longer has external dependency to a local ffmpeg installation, making it a lot easier to get livepeer running in a new environment.