Move user-defined Handlebars helpers to project folder. (lib/)

Essential helpers ("env, "log") are kept to be overwritten/discovered in the project.
Hipsum is removed from xtc's dependencies and added to the package.json in the project generator.
Fix removing of sprites tasks if sprites building is disabled.
Fix tests now that Hipsum is not in xtc itself.
Changed my mind. Hipsum stays. :)
Jup. Add it in json too.