Add a :caller_root option to Garner::Config.

* Use Garner::Config.caller_root to normalize `:caller` values generated
  by the `Caller` context key strategy, for client apps that may be running
  from different places in the filesystem.

[Fixes #29]
Test for exact match on caller spec, and strip leading slashes from caller values
Merge pull request #33 from fancyremarker/29-caller-common-root

Add a :caller_root option to Garner::Config