Defer formatting of log values in Levelf(), Levelln()

The Level() family of log methods defer formatting of log
values until it is confident that the log level needs them.
This change allows that deferral to happen at the Levelf()
and Levelln() levels too.

This commit also adds benchmarks to demonstrate the improved
performance of these functions
Merge pull request #11 from anthonybishopric/defer_formatting

Defer formatting of log values in Levelf(), Levelln()