Handle error messages caused by missing binary libraries.
Improves sudo propmt
Refactor parallel worker code and make it work on jruby
Add parallel worker documentation

Also added --jobs option to man file
Push a fix for SharedHelpers.chdir not being threadsafe

The platform detection is a bit messy and I will refactor it later
Use newly introduced current_ruby method for platform
Start adding specs for bundler parallel install
Add check for JRuby apart from Windows on build_extensions method
Add separate specs for parallel gem installation
Do not use parallel installation on unsupported systems

1. Use parallel installation on MRI if asked.
2. On non-MRI check for rubygems version that is threadsafe
Newer versions of Rails are not installable for old rubies or RG

Install Rails 3.2.13 parallely in test case
Switch from installing Rails to faker & AS
Merge pull request #2567 from bundler/make-paralle-fetch-jruby

Enable parallel fetching for JRuby
we don't actually use those. huh.
Merge pull request #2535 from ixti/issue-1028

Improves sudo propmt
un ghetto-dev sudo endpoint spec
Merge pull request #2019 from luismreis/master

Properly handle missing binary libraries.
Add RubygemsIntegration#provides?
shorten and add spaces to un-threadsafe warning