Only forward attr_accessible if needed

This makes it possible to use acts_as_relation in Rails 4 if
the 'protected_attributes' gem is not loaded.
Use where() and includes() instead of :conditions and :include

Had to rewrite the module_eval to use a block instead of a string
because the options can not be converted to a string reliably.
Remove special cases for older Rails versions

... and print a warning about :conditions and :include
Use default_scope with lambda.
Update documentation
Update README.rdoc

Updated README to include refined overview, installation instructions.
Merge pull request #28 from joshuairl/master

Improved README

Add installation instructions
Merge branch 'rails4' of https://github.com/schuetzm/acts_as_relation
upgrade dummy app to rails 4
conditional test for protected attribs
fix readonly error and N + 1 queries problem
remove Gemfile.lock from gitignore
version bump