Add 'doc' command to groovysh

___OVERVIEW___

This PR is for adding 'doc' command to groovysh. It opens a browser window(s) displaying the doc for the specified class. It is inspired by 'javadoc' command of Clojure REPL. 
I think some improvement possible (local caching, etc.) but it is good enough for startup the feature.

___EXAMPLES___

    doc groovy.lang.GroovySystem
=> opens http://groovy.codehaus.org/gapi/groovy/lang/GroovySystem.html

    doc java.util.LinkedList
=> opens http://docs.oracle.com/javase/7/docs/api/java/util/LinkedList.html (depends on the version of running jdk)

    doc java.util.List
=> opens http://groovy.codehaus.org/groovy-jdk/java/util/List.html and http://docs.oracle.com/javase/7/docs/api/java/util/List.html (depends on the version of running jdk)