Semantic versioning

[enzbus]

Declare support for semantic versioning in Cvxportfolio since version 1.0.0:

https://semver.org/

Maintain a TODOs file with planned changes and which next releases are targeted:

• major release (> 1 yr), possibly breaking changes
• minor release (<= 1yr), new features
• revision or patch (~ 1 mo), bug fixes

Headroom for breaking changes: public API vs private API. Case-by-case basis (e.g., making a private method public is not a breaking change; adding an argument with default value to a public method is not, ...).

Bugs are defined as deviations b/w code and paper or code and documentation. Improving the docs or adding examples count as bug fix.

[coveralls]

coverage: 98.962%. remained the same
when pulling 66e7df2 on semantic_versioning
into b7e3516 on master.