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`
Dosstrings, more performant `sentence_actual`

1) General cleanup of supporting stuff around `sentence` and `sentence_actual`
2) Improved the way sentences are split into words and filtered
3) No longer using `random.shuffle` to yield a random sentence. `random.randint` + list indexing is many times faster.
4) Added docstrings for `sentence` and `sentence_actual`
Merge pull request #2 from TadLeonard/master

Fixed test file, added alternative sentence maker