Martian Proxy v.2.0.0-beta.2

Changes from v2.0.0-beta:

• MessageView: static snapshots of net/http's Request or Response structs. Useful for logging or any functionality when you need to read the full request or response (including the body), but do not want to alter the request or response in any significant way. The martianlog Logger now depends on this.
• Separate out the logging functionality provided by martian into a log package to break cyclic dependency between mitm and martian while allowing them to share unified logging.