scripts: add build, cover and update test

This adds a build script that attempts to be as user friendly as
possible: if they have already set $GOPATH and/or $GOBIN, use those
environment variables. If not, create a gopath for them in this
directory. This should facilitate both `go get` and `git clone` usage.

The `test` script is updated, and the new `cover` script facilitates
easy coverage generation for the repo's constituent packages by setting
the PKG environment variable.
Merge pull request #1028 from jonboulle/build

scripts: add build, cover and update test