Reorganizes asset file structure

Separates assets into
- Source assets: `assets`
- Compiled assets: `dist`

Edits assets.php to reflect changes

Removes '.min' from filenames

Fixes:
- removes 'clean' from build step.
Remove disparity between dev and prod gulp tasks

removes `gulp dev` in favor of `gulp`

adds `gulp --rev` to produce revved asset manifest

adds `gulp --tasks` to the readme

removes postinstall script
removes bower dep
Implement JSON file based asset pipeline

Adds manifest.json which contains all of the mappings for the assets
ref: https://github.com/roots/roots/pull/1138#issuecomment-62593715

Prune and update NPM deps
Use wiredep for sass and less injection

https://github.com/taptapship/wiredep
all third-party deps are injected via wiredep rather than manually managed
Refactor: remove manifest file concerns

https://github.com/austinpray/asset-builder
Moves all manifest parsing concerns to separate node module
Merge pull request #1215 from austinpray/austinpray-gulp

Gulp manifest.json Asset Pipeline