Tightened up the interface - removed the log_path constructor param.

Having both a log_path and logger param is confusing. If a log_path and
a logger param were provided, it was ambiguous as to which would have
been honored. Allowing a logger to be injected is sufficent and makes
for a simpler interface.
Merge pull request #15 from Crowdtilt/remove-log_path

Tightened up the interface - removed the log_path constructor param.