Merge pull request #68 from tsloughter/master

add optoin to use custom system_libs dir. This closes #64
Merge pull request #72 from tsloughter/master

ignore empty list of overlay vars to fix unneeded warning
add a dev mode that symlinks the release instead of copying it

This should only ever be used for development, however it makes it
very, very nice to be able to simple recompile a project without
recopying it to try new things.
Merge pull request #73 from ericbmerritt/master

add a dev mode that symlinks the release instead of copying it
fix load overlay vars to still support single filename not in a list