Improve documentation of Requirement

Remove a bunch of FIX comments that weren't fixed, but were breaking
documentation.
Remove FIX comments

Some of these were creating bogus documentation
Add Gem::Requirement#concat

This does not check for compatibility requirements.
Combine dependencies in RequestSet

If the user gives incompatible requirements the resolver will sort this
out for us.
Ensure incompatible dependencies cause exceptions
Remove extra whitespace
Prevent duplicate sources for the same gem

The bundler Gemfile tests indicate this is not allowed.  Now we raise an
ArgumentError (which is probably not specific enough).
Reduce indentation in gem_path