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
normal sdoc require
Allow forcing main page via env variable

  Ruby source code contains some :main: directive (f.e. in lib/rdoc.rb)
  which overwrite the command line --main option . This commit adds
  the possibility to set a SDOC_FORCE_MAIN_PAGE enviroment variable
  which will overwrite even :main: directives .