Ensure Resque.logger does not return a hash

Not sure if this is a regression caused in an update to resque, but under resque 1.24.1, the resque-logger gem fails to work as Resque ends up trying to call .info on the config hash before it has been converted to a logger object.  A new, formerly failing, spec has been added to the bottom of resque_logger_spec.rb to illustrate the regression.

This pull request updates the ResqueLogger initialization API so that the configuration is stored in a variable called logger_config, with properly named accessor methods.  There may be a way to fix this issue without renaming the configuration method/variable, but I wasn't able to figure out the right path to do so.

Thanks, really love the gem.