opening paragraph 

i restructured the opening paragraph without making major content changes for oine major reason:
1) the most recent version stated the Heidegger's name and birth and went right to influences. Covention suggests that when one is coming to an encyclopedia, one wants to know first who this person is rather than who they influenced. a brief description of who and and what heidgger did should come before anything else, so i changed it. no other substantial change.