Remove class variables from Tabber

@@vars are generally considered a Bad Thing in Ruby.
See http://bit.ly/1kcit9V for a quick rationale.
Fix various `require` and `File.join` calls.

In many cases it's safe to assume the load path will be setup correctly
at runtime, eliminating the need for absolute paths. For instance, in
your bin/synx executable, RubyGems sets the load path such that you can
just `require "synx"`. When you use RSpec, it also sets the load path to
allow you to use `require "spec_helper"`.

I also cleaned up the `File.join(File.dirname(__FILE__)...` stuff to use
the more idiomatic `File.expand_path`.
Merge pull request #36 from alexgenco/refactor_tabber

Remove class variables from Tabber
Merge pull request #38 from alexgenco/fix_requires

Fix various `require` and `File.join` calls.
Remove output from tests

Tabber accepts an :output option, which points to $stdout by default.
Tests set this option to a StringIO object instead.