Updated Doxygen configuration to 1.8.1 https://fedorahosted.org/sssd/ticket/1819

Use HTML_TIMESTAMP instead of HTML_FOOTER_DESCRIPTION https://fedorahosted.org/sssd/ticket/1271

docs: Use absolute srcdir path Building docs only worked in parallel builds. This patch uses abs_top_srcdir to make building documentation work in both parallel and in-tree builds.

Add documentation for PAM response messages

Rename server/ directory to src/ Also update BUILD.txt