Implement basic gemspec support

options handling is not supported, only "gemspec" is loaded, development
group behavior is not handled.
Only add development dependencies when allowed

Now exclusion of development dependencies is supported.
Extracted GemDependencyAPI#add_dependencies

This is used for gemspec dependencies where runtime and development
dependencies which have different implied groups.
Support gem.deps.rb gemspec path: option
Improve gem.deps.rb gemspec name support

Bundler accepts "a.gemspec" or "other.gemspec" for gem "a"
Support gem.deps.rb gemspec name option
Raise when multiple gemspecs are available

The user must pick one with the name: option to #gemspec.
Raise when no gemspecs are available
Raise an exception for bad gemspecs
Hide bad gemspec warning from test