Xlcalculator

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- Add functions: ``ISBLANK()``, ``ISNA()``, ``ISTEXT()``, ``NA()``

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- Add ``COUNTIIF()`` and ``COUNTIFS()`` function support.

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- Add ``SUMIFS()`` support.

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- Updated README with supported functions.

- Fix bug in ModelCompiler extract method where a defined name cell was being
overwritten with the cell from one of the terms contained within the formula.
Added a test for this.

- Move version of yearfrac to 0.4.4. That project has removed a dependency
on the package six.

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- In-boarded xlfunctions.

- Bugfix COUNTA.

* Now supports 256 arguments.

- Updated README. Includes words on xlfunction.

- Changed licence from GPL-3 style to MIT Style.

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- Make dependency resolution part of the execution.

* AST eval'ing takes care of depedency resolution.

* Provide cycle detection with reporting.

* Implemented a specific evaluation context. That makes cache control,
namespace customization and data encapsulation much easier.

- Add more tokenizer tests to increase coverage.