Overhaul image submodule

The `skfuzzy.image` submodule had very low test coverage, less-than-ideal naming conventions, and redundant functionality also seen elsewhere in the package.

Summary of changes
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* Add astronaut test image for image operations
* Replace old, limited `pad` function with new version in line with NumPy >= 1.8 (included here directly to avoid dependency creep, because while it was introduced in NumPy 1.8 it is backwards compatible with earlier versions)
* Refactor `pad` out of `skfuzzy/image/shape.py` into its own file, `skfuzzy/image/arraypad.py`, for maintainability
* Combine `contrast` with `focus`; the latter was incorrectly named. They both did contrast adjustments. `contrast` tests updated to cover new functionality.
* Rename `defocus` -> `defocus_local_means` which more correctly identifies its functionality
* Remove `sinmse` algorithm, which was not working as intended
* Full test coverage

Changes outside image submodule
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* Update Makefile to use safer local paths
* New `make` target, `clean-cov` for removing directories that can be created by coverage, including `./htmlcov`
* `__init__.py` removed from `control` submodule. Todo: clean up, annotate, and refactor these files into doc/examples
* `imcontrast` renamed `sigmoid` and moved to `fuzzymath` submodule, as it is n-dimensional capable - not limited to images - nor is it the only or best contrast adjustment available (see updated `contrast` function)
* PEP8 fixes
* Documentation clarifications

There is some significant API breakage in this PR. I believe now is the time to do it, but **please speak up if these changes would create major problems for you**.