Add optional logger to log every request

Just the possibility to set a logger for the Linode instance, either through initialize or the accessor. If it is given, then use it to log every single request. Nothing changes if it is not given.
add a spec for passing in a logger
use accessor since we have it
bring README.md up to date
Merge pull request #22 from rick/more-work-on-logger

Land #19 (with additional tidbits)