Added acquire_ticket_vm API to retrieve a screen ticket for VMRC browser plugin

API Documentation:
http://pubs.vmware.com/vcd-55/index.jsp#com.vmware.vcloud.api.doc_55/GUID-0DF63045-16D7-49E7-878E-B56E27F99C62.html
http://pubs.vmware.com/vcd-55/index.jsp?topic=%2Fcom.vmware.vcloud.api.reference.doc_55%2Fdoc%2Foperations%2FPOST-AcquireTicket.html

VMRC documentation:
http://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vs5_1_vmrc_prog_guide.pdf
https://www.vmware.com/support/developer/vmrc/