add jruby_18 and jruby_19 to platforms
adding platform support for ruby 2.1

* added new platform and ruby version helpers
* updated man file and tests
Merge pull request #2647 from brandonblack/ruby-2.1-support

Adding platform support for Ruby 2.1
Merge pull request #2611 from mcfiredrill/jruby18and19

add jruby_18 and jruby_19 to platforms
extract into Bundler.ruby_version
add Bundler::RubyVersion#host
include the unknown engine's name
add user agent to fetcher requests
also include host data in the user agent
on JRuby, RbConfig::CONFIG["host"] doesn't exist
default requests have "Ruby" in the user-agent
init the connection once
set user agent without "Ruby, "
random id to correlate requests in one run
Add command to user agent for fetcher requests
Checks bundler, rubygems, ruby, command