Fixed test file, added alternative sentence maker

1) Fixed usage of `number` in tests (it takes one arg, not two)
2) Added `sentence_actual`, which returns an actual sentence from the Sherlock text.
3) Added `_picka._Book` class to hold the text and split sentences read from Sherlock.
   Users can call sentence() without reading the entire file again and again.
4) Added test of `sentence_actual` to `picka.tests`