Support not using railtie

* Appraisals: new file
* Gemfile: updated
* NEWS.rdoc: updated
* README.rdoc: added info on using SafeAttributes::Base
* Rakefile: updated, no more jeweler
* VERSION: removed
* gemfiles/rails-3-0.gemfile: managed by appraisal
* gemfiles/rails-3-0.gemfile.lock: ditto
* gemfiles/rails-3-1.gemfile: ditto
* gemfiles/rails-3-1.gemfile.lock: ditto
* gemfiles/rails-3-2.gemfile: ditto
* gemfiles/rails-3-2.gemfile.lock: ditto
* lib/safe_attributes.rb: refactored into SafeAttributes::Base
* lib/safe_attributes/base.rb: new file
* lib/safe_attributes/version.rb: new file
* safe_attributes.gemspec: manually updated, no jeweler
* spec/safe_attributes/safe_attributes_spec.rb: updated some tests
to verify new functionality works as expected