Remove ActiveSupport dep., deprecated attr_accessor_with_default

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Avoid absolute paths with require where possible

* Without File.expand_path, this breaks on Ruby 1.9.
  Regardless, Rake::TestTask is configured to set-up LOAD_PATH for us.

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Fix tests and get rid of deprecation warnings.

Also, this commit makes tests compatible with rake 10.