Convert to idiomatic-gulp approach

The plugin now returns 3 streams:

* main stream which includes both css and image files
* "img" stream (only images)
* "css" stream (only css)

The code should still be backward compatible.

This fixes two problems:

1. Now the output can be piped into other plugins.
2. In the previous technique gulp can never tell if the task has
   actually completed because it doesn't have a stream to test.

   See orchestrator's runTask for more information:
   https://github.com/orchestrator/orchestrator/blob/master/lib/runTask.js
Moving to `master` pipe as default example
Starting on individual stream modification
Merge branch 'master' into pr/16.readme
Fixed output directory
Added gulp-csso and gulp-imagemin to example
Updated dependencies
Moved to master stream inside of docs tests
Added pipeline example to runnable script
Accidentally over defined tasks, fixed now
Reenabled optimizers
Ignoring example outputs
Correcting dependency ordering
Re-ordering dependencies via `npm install`
Correcting documentation notes
Added returns section to documentation
Making stream name consistent with other documentation
Release 1.2.0