By Denny Dyroff, Entertainment Editor, The Times John Mayall If you’re a fan of blues music, the only place you should want to be on August 20 is the Sellersville Theater (24 West Temple Avenue, Sellersville, 215-257-5808, www.st94.com). The two acts on the bill, both of whom specialize in the blues, have been making music for a combined total of 118 years.    Send article...