Dependency support

[lpetre]

I'm trying to setup a custom buildpack that can provide libzmq if necessary. The issue I ran into is that libzmq requires uuid-dev, which is in utils-linux. I can use vulcan to build utils-linux, but then was faced with the question: how do I get that version of utils-linux onto the machine that is building libzmq.

Rather than setup a custom buildpack for vulcan that included utils-linux, I added an option to vulcan build that lets me include any number of vulcan built dependencies. These dependencies get pulled down via http and expanded into a deps folder, which is then added to the LDFLAGS/CFLAGS/etc environment variables.

This also includes my other pull request, to fix the cradle errors.

[tjwebb]

When I try to use this, I simply get this output:

$ vulcan build -v -d "https://s3.amazonaws.com/gisio-geoserver/tools/fuse-2.9.vulcan.tgz"
Packaging local directory... done
Uploading source package... Internal Server Error!! Unknown error, no build output given

[tjwebb]

Wow finally figured this out. This works:

vulcan build -v --deps '["https://s3.amazonaws.com/gisio-geoserver/tools/fuse-2.9.vulcan.tgz"]'

This is documented precisely nowhere. But although it downloads my dependency, it doesn't seem to work. That is, I don't think it's being properly extracted and installed. Can someone comment on this?