Ability to override the Template caching providers

[jonathanaustin]

At the moment, if projects want to use WTemplate they are required to have a jsr107 caching provider (eg ehcache).

Caching can be disabled to avoid this dependency, but if a project wants to use cached templates but not a jsr107 provider then they need to be able to override the default caching provider.

Each rendering engine (ie Velocity and Handlebars) handle caching differently so each rendering engine will need this ability to override the default caching provider.