Boston’s Huntington Theatre Company is losing its artistic director. 68-year-old Nicholas Martin, who joined the company in 2000, will hang it up in 2008, and assume the title of artist emeritus for two additional seasons. “During his tenure in Boston, the Huntington built two theaters in the South End and launched a play development wing. And Martin’s ties to New York and to Williamstown brought in a stream of both big name and promising young actors as well as national attention.”