Move address manager to its own package.

This commit does just enough to move the address manager into its own
package.  Since it was not originally written as a package, it will
require a bit of refactoring and cleanup to turn it into a robust
package with a friendly API.
First round of address manager package refactor

These changes are a joint effort between myself and @dajohi.

- Separate IP address range/network code into its own file
- Group all of the RFC range declarations together
- Introduces a new unexported function to simplify the range declarations
- Add comments for all exported functions
- Use consistent variable casing in refactored code
- Add initial doc.go package overview
- Bump serialize interval to 10 minutes
- Correct GroupKey to perform as intended
- Make AddLocalAddress return error instead of just a debug message
- Add tests for AddLocalAddress
- Add tests for GroupKey
- Add tests for GetBestLocalAddress
- Use time.Time to improve readability
- Make address manager code golint clean
- Misc cleanup
- Add test coverage reporting