Improve assets build pipeline

[francisbrito]

By moving our "processable" files (.scss, .coffee, ES2015, etc.) into a separate folder we no longer are forced into committing processed files into version control.

In addition, 2 gulp tasks were added:

• gulp copy-tasks which copies all files except processable ones in assets into the "out" directory .tmp.
• gulp build:scss which, in addition to doing the same as gulp styles is optimized to only compile files that have changed since last run. (and its name makes its purposes clearer)

[francisbrito]

By the way, this fixes #11 @eatskolnikov

[francisbrito]

Merging this.