Add a --force option to protect against accidental deletion of validators

It's hard to recover an org from having a deleted validator client -
this patch protects against deleting validator accidentally e.g. in a
for loop around knife client delete by requiring you to specify --force

Includes specs around this new functionality
Merge pull request #1272 from opscode/jc/OC-130/knife-delete-validator

Add a --force option to protect against accidental deletion of validators