Add development dependencies to gemspec, and add Gemfile

Only rspec is really required for development, as tzinfo,
activemodel, and nokogiri are only need to test specific
extensions/plugins.  But it is best to install all of them
so that all plugins/extensions are tested.

Author: jeremyevans

Gemfile

@@ -0,0 +1,2 @@
+source 'https://rubygems.org'
+gemspec

sequel.gemspec

@@ -17,4 +17,8 @@ SEQUEL_GEMSPEC = Gem::Specification.new do |s|
   s.require_path = "lib"
   s.bindir = 'bin'
   s.executables << 'sequel'
+  s.add_development_dependency "rspec"
+  s.add_development_dependency "tzinfo"
+  s.add_development_dependency "activemodel"
+  s.add_development_dependency "nokogiri"
 end