set $MANPATH in Bundler::Runtime#setup_environment

This lets `bundle exec man ...` show man pages from bundled gems.
Handle error messages caused by missing binary libraries.
Add ENV[DEBUG_RESOLVER_TREE] which outputs the resolver tree when bundle install.
Add a spec for issue #2124 (I think)
Fix for issue #2124

 - don't complain if local_override being updated anyway
Arguably a cleaner fix for issue #2124
Note change in changelog
Fix gem_helper_spec.

Fixes "Bundler::GemHelper tasks gem management release releases even
if tag already exists", otherwise the test case fails with:

     RuntimeError:
       There are files that need to be committed first.
remove quotes on RUBYOPT on Windows
test against rubygems 2.0.2
use Gem::RemoteFetcher directly

this means that the automatic SSL in Rubygems 2.0.2 no longer
breaks things for us, because we don't use it. this also makes
fetching slightly faster, since the marshalled array is no
longer converted into Gem::NameTuples and then back to arrays.
rescue prerelease errors correctly, oops
actually concat pre specs
require rbconfig, fixes #2382
Merge pull request #2376 from ogra/rubyoptwin

remove quotes on RUBYOPT on Windows
remove quotes on RUBYOPT on Windows
Require minitest_helper properly in example test file.
Write skipped groups to output for bundle install and update actions.
remove specs duped in post_bundle spec
without groups message in sentence form