Add new version defines, update ofGetVersionInfo().

New defines are OF_VERSION_MAJOR,  OF_VERSION_MINOR, OF_VERSION_PATCH. Old define OF_VERSION is removed because of incompatibility with new system (old MINOR is new MAJOR). This way any code that uses it fails to compile instead of silently making wrong decisions.
Add info about SemVer to the global Readme.

This closes the OF-repo side of #1706.
Remove the prefix text in ofGetVersionInfo().

This removes the unnecessary text which should be added at the user side anyway, and makes it easier for people to parse the OF version string, to compare against for compatibility etc. 
ofGetVersionString() would be a better name, but it's not worth changing the API for this, imo.
Merge pull request #1811 from bilderbuchi/feature-semver-changes

Semver changes