set $MANPATH in Bundler::Runtime#setup_environment

This lets `bundle exec man ...` show man pages from bundled gems.
Add ENV[DEBUG_RESOLVER_TREE] which outputs the resolver tree when bundle install.
Run gem installation in parallel
Add "--jobs, -j" option
Prohibit Dir.pwd while another thread is doing Dir.chdir
Improve messages for multi-threaded installation
Don't start additional thread if jobs == 1
Remove unnecessary chdir to improve concurrency
Fix to use SharedHelpers.pwd
Run installation in multiple processes unless host_os is Windows
Move constants to use it from SharedHelpers
Report exception to parent process
Fix to report post_install messages
Display Gemfile dependency info in bundle outdated output
Output outdated gems specified in Gemfile before those only in Gemfile.lock
Implement Dependency#specific? for use with rubygems <= 1.7.2
Set Thread#abort_on_exception locally
Kill worker process when it is requested to stop
Obtain gemspecs from the local system cache if available.
Prefer file:// gemspecs over cached gemspecs