Add project/build.properties

This project won't work with SBT 0.13, at the present state.
project/build.properties should always be there, so that the SBT
launcher knows which SBT to use.
Target 0.13.0, not -RC4

Avoid needing to fetch an RC for no good reason.
Merge pull request #9 from Blaisorblade/topic/build

Update build config