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