move mongoid/dm gems into Gemfile and load dynamically based on MODEL_ADAPTER env variable

Gemfile

 source "http://rubygems.org"
 gemspec
+
+case ENV["MODEL_ADAPTER"]
+when "mongoid"
+  gem "bson_ext", "~> 1.1"
+  gem "mongoid", "~> 2.0.0.beta.19"
+when "data_mapper"
+  gem "dm-core", "~> 1.0.2"
+end

cancan.gemspec

@@ -15,10 +15,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rr', '~> 0.10.11' # 1.0.0 has respond_to? issues: http://github.com/btakita/rr/issues/issue/43
   s.add_development_dependency 'supermodel', '~> 0.1.4'
 
-  s.add_development_dependency 'mongoid', '~> 2.0.0.beta.19'
-  s.add_development_dependency 'bson_ext', '~> 1.1'
-  s.add_development_dependency 'dm-core', '~> 1.0.2'
-
   s.rubyforge_project = s.name
   s.required_rubygems_version = ">= 1.3.4"
 end