Pylytics

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- Using a fork of Oracle's MySQl adapter, which is hosted on PyPi.
- Updated docs - a recent version of pip is required to install pylytics
properly.

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- Cleaned up README - most of it was duplicated from the main docs.
- Added --allow-external line to requirements.txt, so it can be installed
locally via pip without additional flags.
- Removed unused crud from setup.py.

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- Historical dimensions - a __historical_source__ can now be specified for
Dimension subclasses, which will be tried first when doing a historical update
of a Fact. If it's not available, it will fall back to __source__.
- Cleaned up tests - removed some old cruft. Added tests for unique constraints
on tables.
- DegenerateDimension - providing this new column type for storing dimension
values in the Fact table, rather than using a foreign key to a separate
Dimension table. Used sparingly, it's very useful.

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- Using unicode for the main internals.

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- All natural keys in dimensions have indexes now - massive performance
improvement for inserting fact rows.

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- Fixing error in requirements file. Only latest MySQL adapter is hosted by
Oracle.