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: report cmd timeouts as 'error'

Instead of just 'failure'
git-build: report errors in footer

like timeouts and merge failures.