Add geotools maven repository to conscript.
Add mapfish repository to conscript.
Add opengeo repository to conscript.
Add gt-tool script to replace conscript gt-tool.

This gt-tool script downloads the task binary from a cache in
.geotrellis or in the local sbt target directory.
Properly concatenate reference.conf when building assembly.
Merge pull request #409 from joshmarcus/topic/gt-tool-fixes

Add script for running gt-tool binary.