Merge pull request #64 from SIPp/gtest

Add Google Test framework
Remove inline gtest.
Add gtest via git submodule.

(And move excess -I includes to sipp_unittest only.)
Merge pull request #66 from ossobv/wjd-no_included_gtest

Fix #65: don't include gtest sources.
Merge remote-tracking branch 'origin/master' into auth-int

Conflicts:
	Makefile.am
	Makefile.in