WIP: Added stub/draft of sbuild scheme handler.
Added "sbuild" scheme with "project" and "now" paths.
Moved md5 sum calculation into separate object.
Added property checker to sbuild scheme handler.

With "sbuild:prop=MY_PROP" you can depends on the property MY_PROP. Whenever the property changes, the dependency will promote a new lastModified timestamp.
Merge branch 'master' into sbuild-scheme
Merge branch 'sbuild-scheme' into 'master'.
Disabled SBuild 0.7.5 in travis again.