Minor fix to README

It should point to SciRuby's fork of statsample.
Removed BioRuby-related comment

This comment is unnecessary as this is a general scientific gem.
Use bundler's gem_tasks to build gem

This commit does a bunch of modifications to the gemspec and the
Rakefile:

  - Removes unnecessary dependencies, i.e. bio, bundler and jeweler.
  - Add rake as a dependency.
  - Add a rake task to initialize an IRB console with
    statsample-timeseries loaded.
  - `require 'rubygems'` isn't really necessary anymore.
Ops, shouldn't have removed bundler dep
Fix require in rake task
Merge pull request #15 from agarie/use_bundler_gem_tasks

Use bundler gem tasks