Joe Strummer was a mensch. That's one thing the makers of "London Town," which opened Friday in New York and Los Angeles, want you to know about the late frontman of The Clash. Set in 1979 in working-class London, the film, directed by Derrick Borte ("The Joneses") and written by Matt Brown, Sonya Gildea and Kirsten Sheridan ("In America"), is a coming-of-age story laced with the political upheaval circumstances of the time: the rise of Thatcherism, hate