Support relative path to build scripts, rather than urls

Hey there. I don't want to have to upload every build script onto the net just so anvil can download it again. I push my build script to the anvil instead, and have it download the sources it needs directly from s3. I had to teach anvil-cli to accept a relative path (see pull request on anvil-cli: https://github.com/ddollar/anvil-cli/pull/8). 

I also had to ignore the Content-Length header sent by anvil-cli? Stat-ing the file size seemed to work better otherwise onward uploads to s3 exploded. What am I missing?