Add support for --json-attribute option

Passes through a JSON string to be added to the first run of chef-client
when using the --bootstrap option for `knife vsphere vm clone`
Merge pull request #30 from skymob/json_attribute_support

Add support for --json-attribute option
merged in patch to add option disabling customizations (from
https://raw.github.com/sacx/patches)
Added VM Snapshot functionality