Allow RDoc ~> 4.0.0.rc
First pass to make sdoc work with RDoc 4

These are the bare minimum required changes to make the generator pass without errors. Some
things still not working:

* Github link now showing up
* Code blocks not working correctly (using Markdown)
* Probably more errors
bump rdoc to released 4.0.0
default to RDoc::Options#pipe = true
add Gemfile, just points to sdoc.gemspec
add development dependencies
remove rspec task and add rake testtask
added specs for SDoc::Generator using MiniTest::Spec::DSL with RDoc::TestCase
update .gitignore
fixed search index option with @KevinSjoberg
Rename the search index option template since being changed in 59e97cc
Remove version specification from 'gem' definition in lib/sdoc.rb

This specification is unnecessary because we already specify the version
dependency in sdoc.gemspec
Remove unused TestCase subclass

We were going to use this to implement RDoc::TestCase features in the
MiniTest::Spec::DSL, however TestCase won't work this way and will have
to reconsider another way.
Move spec/sdoc/* to spec/*

Since we only have a one spec file right now, I'm just going to keep
them all under ./spec
Update Rake::TestTask to reflect re-organization of spec folder
Use RDoc::TopLevel#last_modified instead of #file_stat.mtime
Move Rails search_index template file to match generator changes
Use RDoc ~> 4.0 and < 5.0