Added negative matcher tests
Modified matcher to only handle one attribute at a time

Tests end up more explicit this way. It is also a somewhat more
canonical approach to matchers in general.
Moved matcher to strip_attributes/matchers

i.e. Shoulda isn't required to use the matcher
e.g. Minitest-Matchers could be used instead
Added rdoc to the strip_attribute matcher
Updated README to reflect new matcher support
Deprecated shoulda macros in favor of matchers