zero length should not have units assigned

lengths of 0 do not require any unit assigment
Use sass value helpers in the cli extensions.
fix/update test expectations

addressing @chriseppstein 's remark.

all tests pass when running `bundle exec rake test features`
zero length should not have units assigned

lengths of 0 do not require any unit assingment
adding test case + expectation

adding since I failed to find a test case for table borders mixin
Use the new sass value helpers for sass value creation in the core extensions.
Move Sass Value class references to the new Value module.
Move tree node references to the new tree module
Use the sass declaration helper.
Fix some bugs in the watcher when files were added and removed.
Merge remote-tracking branch 'mprins/patch-2'
Fix broken tests
Sourcemaps!
Don't recompile files that don't have a sourcemap unless the file is supposed to have a sourcemap.
Bump version to 1.0.0.alpha.16.
Bump import-once version.
Bump version to 1.0.0.alpha.16.
test sourcemaps and change the filename to match the convention used by sass.
Make sure timings still work now that we have sourcemaps.
Cleanup the integration test projects better.