Add debug group (pry-debugger/pry-stack_explorer)

Add:
 * Debug group to Gemfile (pry gems)
 * Require the debug group on the specs (for debugging tests)
 * The exclusion of the debug group in Travis CI
Add run task and ability to specify a target git repo for the CLI

Add:
 * A 'run' task which specifies a git repo to run git_statistics on with
   debugging enabled
 * The support for a specifiable git repo in the executables and the CLI
 * The default support for using the present working directory if one is
   not specified