Ensure that g:unite_source_session_path is set before accessed

Recent updates to unite led to an error in unite-session:

    :Unite se...                                                                                                                                                                                                                                   
    Error detected while processing /home/housl/.vim/bundle/unite-session/autoload/unite/sources/session.vim:                                                                                                                                      
    line   37:                                                                                                                                                                                                                                     
    E121: Undefined variable: g:unite_data_directory                                                                                                                                                                                               
    E116: Invalid arguments for function unite#util#set_default                                                                                                                                                                                    
    function <SNR>35_call_unite_empty..unite#start..unite#start#standard..unite#candidates#_recache./ <SNR>137_recache_candidates_loop..<SNR>137_get_source_candidates..177, line 1                                                                 
    Vim(let):E121: Undefined variable: g:unite_source_session_path                                                                                                                                                                                 
    [unite.vim] Error occured in gather_candidates!                                                                                                                                                                                                
    [unite.vim] Source name is session  

unite-session should probably call unite#get_data_directory() instead of assuming that the global variable is set, correct? 

Thank you for the awesome plugins.

Jason