Add generic conversion formulas support, stable release, fully backwa…

…rd compatible.

Conversions seem work, stable release, backward compatible.

Stable release, fully backward compatible.

First stable relase, API based on UOM parser, power symbolic math on …

…units symbol, not backward compatible.

SI units system almost complete. Unstable release, but backward compa…

…tible with v0.0.4.

The 4 basic operators, +-*/, have a first functional version. A comph…

…rensive tests suite is necessary and will come alongside the grows of the SI system.

Unstable release, not backward compatible.

Modify API design:

+ add *parents* units (e.g. length, time...)
+ units are children of *parents units* (e.g. length => meters, kilometers, feets)

Unstable release, not fully backward compatible.

Operators `+` and `-` first time functional, unstable release, not fu…

…lly backward compatible.

First release to test classes design, pre-alpha release.