Spanwer default values as constants

Updated all references of the spanwer properties default values to use
contants.
Small fix of the 'summon' command
Polished the code, added JSON integration

JSON.Stringify is now used to create the commands, custom objects will
be created to reflect in-game NBT values.
CSS file
Improved stringify integration, mob classes and bug fixes

- Added new function to customize JSOn.Stringfy result;
- Added SpawnData to mob spawners;
- Created mobs basic classes, with support for presets;
- Improved creation of the mob list;
- Changed mob selector, now a hidden field is used to save the
selection;
- Internal objects now make use of the toJSON method for complex JSON
output customization;

- Fixed: attachement of the generateCommand was causing the command to
be fired;
- Fixed bug with inheritance - NBTInfo of the mob classes was being
shared by multiple instances;
CSS path fix
Changelog update