Register looked-up adapters/accessors in their own right

DataAccessor and InstanceAdapter classes are looked up for derived types
by inspecting the type's ancestors and searching for registered adapters
up the chain.

When loading a large number of facets this causes the lookup to be
invoked many times with the same arguments. By registering the located
adapter or accessor with the derived type, this is avoided for all calls
but the first.
Test that adapters found by ancestor lookup are registered.
Added support for empty arrays in restrictions.

This allows your timid code to go from looking like this
  with(:my_field, some_values) if some_values.present?
to become confident code like this
  with(:my_field, some_values)
Test adapter/accessor memoization using state checks
Clearer method naming in adapter lookup
Search all ancestors in adapter lookup
add license information to gemspec

this way it can be used with rubygems.org API
Merge pull request #359 from jordimassaguerpla/master

add license information to the gemspec
update jars and wars to solr 4.1.0
update jetty-specific config files
convenient install task for development
set version to 2.0.0.pre.experimental.solr4
try this with default logging
add context descriptor
update solr configuration to work with solr 4.1.  Most notable change is that the default core moves from /solr to /solr/default, due to the requirement in Solr 4+ to be multicore by default
fix typo
Update the Rakefile to run the tests for the current versions of Rails that we're testing against.
Sunspot 2.0.0.pre is now 2.0.0
Made empty array trigger a query equivalent to a no-op. Added documentation about the behaviour.
Merge pull request #334 from culturecode/empty-array-support

Added support for empty arrays in restrictions.