build: use configed flux-core cmd before installed

We were appending the configured flux-coure source directory's
src/cmd directory to PATH and calling 'flux'.  An installed
copy of 'flux' would take precedence when we want the configured one.
build: elminiate gcc-4.8.2 compiler warnings

Newer gcc is pickier than the one we use on rhel 6.
Merge pull request #6 from garlick/build-fixes

couple of minor build fixes
Initial draft of README.md
Merge pull request #7 from lipari/add-readme

Add README.md to flux-sched