Use bundler gem tasks

With this PR, you'll be able to build and upload this gem to Rubygems with the `rake release` command. Also, I removed some extra dependencies (`bio`, `jeweler`) and added `rake`.

Ah, there are some small fixes as well:
- The statsample URL should point to SciRuby's fork.
- Removed bioruby-specific comment.
- `require 'rubygems'` isn't necessary anymore (since Ruby 1.9).

I also added a `rake console` command that'll start an IRB shell with `statsample-timeseries` already loaded. It's good when you're debugging.