Add tag util.player_is_valid[name]

A rather useful tag for checking name validity.
Also fix a meta typo.
Fix some dCuboid issues, fixes #768

- dCuboids read wrong when input from a tag due to escaping - handle
that neatly.
- steps on smart event didn't allow cu@ in names
Gamemode mechanism

adding gamemode mechanism things
gamemode mechanism fix

fix
Add entity.can_breed tag/mech

A way to read and adjust whether an entity is capable of breeding.
Also, add a note in some world event meta
Add entity.is_breeding,breed

Yay for more NMS obfuscated code @.@
Update readme, add false option to breed mech

- breed mech needed a 'breed:false' option
- readme link was outdated
Merge pull request #776 from mcmonkey4eva/master

Pull request for NMS Obfuscated junk