PEP 257!
Interface changes (details below)

- start_vm & stop_vm return True if a VM is already in the requested state instead of Excepting out
- delete_vm will stop a vm before deleting it instead of Excepting out
- Methods that aren't implemented should now have Exception messages for pretty-printing by CLI tools that use these classes
Merge pull request #221 from seandst/cleanup-vsphere-provider-list

Cleanup vsphere provider list