Adding message serialization for new pam_escalate.so module
Update README to describe pam_escalate.so and why it should be used.
Added unit-tests for escalate_message.c.
Added escalate_helper.{h,c} which is built into pam-escalate-helper.
Added escalate_subprocess.{h,c} for launching and talking to pam-escalate-helper.
Added escalate_module.{h,c} which is built into pam_escalate.so.
Updated build system to include pam_escalate.so and pam-escalate-helper.
Updated .gitignore with new files for pam_escalate.so.