Add a singleton logger, convert puts statements to log statements

Current approach will display log messages along with the severity, file
path from the lib directory, and line number. For example:

"DEBUG [git_statistics/collector.rb:111]: No files were changed"
Add log default ERROR level, refactor verbose log level

Add:
 * Default level for the logger, ERROR
 * An INFO log statement when extracting commits

Refactor:
 * Old verbose logging approach to now adjust the logger's level to INFO
 * Specs to reflect missing verbose option from collector class
Add debug flag option, improve flag description for verbose
Trim log format on verbose/normal use, add 'use_debug' option on logger

Add:
 * Conditional approach to displaying full logger information only when
   we're debugging
Add initial Log specs, add splat as parameter to Log#respond_to_missing?

Add:
 * Initial Log specs
 * A naked splat as a parameter to the Log#respond_to_missing? so that
   the rspec logs provide the expected results on a failing spec.
   Otherwise it was just complaining about the wrong number of arguments
   on all failing specs.
Refactor splat in Log#respond_to_missing to use parameter with default
Put Log class in the GitStatistics module
Merge pull request #43 from kevinjalbert/add-singleton-logger

Add singleton logger