Simple interchangeable evaluation engines.
Allow for NaN fill values in stack/mosaic.
Update README.rst
Bump version to 0.4.0
Bump version to 0.5.x
Add tests
Small tidy up
Merge pull request #58 from rhattersley/backend

Simple interchangeable evaluation engines.
Provide Engine.ndarrays() and Engine.masked_arrays().

Closes #61.
Thread-per-node evaluation
Remove obsolete engines
Docstrings and minor refactor
More comments and minor refactor
Switch to true-division
Remove obsolete abstract methods
Promote array/axis to AggregationStreamsHandler
Correct axis limitation notes
Another docstring
Fix negative axis handling and comment typo
Better error handling