build-log: make optional and only use streams

* Make build logs truly optional
* use streams for simplicity
   * Also fixes the missing 'log started' header
Merge build-log--fix-header (#50) into master

build-log: make optional and only use streams

* Make build logs truly optional
* use streams for simplicity
   * Also fixes the missing 'log started' header
git-build: don't munge log urls

path.join() is onyl for filesystems and was collapsing sequential '/'
chars leading to urls like 'http:/blah.com/blah'. Doh!

Alos, add tests.