Add jenv plugin

Initializes jenv and provides the jenv_prompt_info funtion to add
Java version information to prompts.  This function is stubbed in
prompt_info_functions script to allow it to be safely called
regardless of whether or not the jenv plugin is loaded.

It also splits detection of the plugin/versions directory and bin directory
to suppport the way Homebrew splits the jenv bin and data directories

lib/prompt_info_functions.zsh

@@ -12,7 +12,7 @@
 # Real implementations will be used when the respective plugins are loaded
 function chruby_prompt_info hg_prompt_info pyenv_prompt_info \
   rbenv_prompt_info svn_prompt_info vi_mode_prompt_info \
-  virtualenv_prompt_info {
+  virtualenv_prompt_info jenv_prompt_info {
   return 1
 }