The 2009-'10 theater season officially kicks in this weekend with the opening of the Rep season. Amadeus, Peter Shaffer's 1979 drama about one man's attempt to bring down God, is sure to provide sumptuous sets and costumes and lots of thrilling Mozart music (played not too loudly, please). But there's another offering this weekend that, on its own modest level, is highly intriguing. The Theatre Guild of Webster Groves opens a two-week run of Samuel Taylor's rarely seen romantic comedy, Sabrina F