add 'doc' command for groovysh
Incorporated review comments from Andre Steingress
GROOVY-6150: small tweaks, new tests
fix a test fail, clean code a bit
do not depend on a version of Java SE
Merge pull request #182 from masatonagai/feature/groovysh_doc_command

GROOVY-6150: Add 'doc' command to groovysh