Define a SPIDEROAK_GIT_DIR env variable and add some doc to README.md

In order to make it easier to clone the SpiderOak repositories to a location other than $HOME/git, I defined a SPIDEROAK_GIT_DIR environment variable that defaults to $HOME/git if it isn't defined.  I also added some comments to the README.md file describing all the environment variables I found in the src and test directories.

One last thing I added is an env.sh script that I source in when I am working on gospideroak.  This just makes sure that GOPATH and SPIDEROAK_GIT_DIR are defined and makes sure the standard go directories of bin, pkg, and src exist.  This is just a convenience script that I use that I thought you might find useful.