Added g_mime_gpg_context_get/set_use_agent()

2011-06-08  Daniel Kahn Gillmor  <dkg@fifthhorseman.net>

	Fix for bug #651826

	* gmime/gmime-gpg-context.c
	(g_mime_gpg_context_[g,s]_set_use_agent): New functions to allow
	the invoker to declare that they expect there to be a
functioning
	gpg-agent, and want gpg to talk to it for any needed
credentials.