Nfldb

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- Update nflgame dependency to get playoff games.

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- Fix time handling for games starting at 12pm/12:30pm.
- Depend on nflgame >= 1.2.9.

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- Depend on nflgame >= 1.2.8 (see nflgame95).

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- Fixed a bug that was preventing `nfldb.aggregate` from working.

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- Added a new "simulation" mode to `nfldb-update`. Try it with
`nfldb-update --simulate 2014081501`.
- Added a new `game_date` field to plays that corresponds to the
first 8 characters of the play's game gsis_id. This ends up
translating to the date that the game played, which just so happens
to be how play identifiers are handed out on NFL.com. This makes
sorting plays with respect to *when* they happened simple.
- Refactored some of nfldb-update into its own module so it can
be invoked easier from other programs.
- Better error messages when sorting with invalid field.

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Minor refactoring and performance features.

The refactoring is mostly confined to the SQL 'ORDER BY' generator. I
had intended to make it more powerful, but that turned out to be more
work than I expected. Instead, I'll settle for clearer code.

The performance features aren't likely to be useful unless you're doing
heavy analysis or are using nfldb to power a web site. Briefly, several
`fill` functions have been added to nfldb's entity types that will, for
example, fill in a list of plays with drive and game data using as few
queries as possible. Currently, this will fall over with >5000 items
because of PostgreSQL limits, but future work will fix this using some
sort of batching mechanism.