Add parameters to customize vmware-tools yumrepo.

Added yum_server, yum_path, just_prepend_yum_path, priority, protect,
proxy, proxy_username, and proxy_password parameters.
$just_prepend_yum_path allows the user to *prepend* an overridden
$yum_path onto the the default ./esx/latest/rhel6/x86_64/ path instead
of replacing it.
Rspec tests included.
Update README for yumrepo parameters.
Merge branch 'feature/yumparam' into develop
Update rake and Travis testing environment.