testsuite: add src/common/libtap

Brought in tap.c and tap.h from https://github.com/zorgnax/libtap
Grabbed the latest master - last modified 2014-05-15.
testsuite: convert nodeset unit tests to TAP

Create test/nodeset as a check_PROGRAMS target.
This is nodeset.c compiled with -DTEST_MAIN.
nodeset_selftest() was converted to a conditionally compiled main().
testsuite: run unit tests from their Makefile.am
testsuite: build libtap as a check_LTLIBRARY
Merge pull request #43 from garlick/libtap

import libtap, convert nodeset tests to TAP