more caching in vmbix

Added more caching options in Vmbix. Now you can control cache size and ttl, or you can disable it by setting ttl to 0. Using cache can improve CPU usage on vcenter, see attached screenshot. 
Also added:
- {#UUID} macro to datastore discovery
- {#CLUSTER} macro to datastore and esxi discovery
![vcenter-cpu-usage](https://cloud.githubusercontent.com/assets/10206601/5569696/652b2b0e-8f75-11e4-96f5-a9bab67fa02a.png)