Introduce task abstraction - choose tasks to run

Relevant for #10
Still work todo on abstraction of tasks. Tests bad

Working on #10, still not ok
All tests are passing in new api, see #10
Extract functions from main file.

See #10
Update Readme
Minor changes to readme again for new version
fix readme and add autoprefixer test
0.5.0
Add autoprefixer example
Add option to specify tasks that output to dir

See #10.

For example: filerev needs dest as a directory
This will require fixing the output of the optimized jade file
#10 - filerev works without optimized jade
Fixes #10 - grunt filerev is working

Changes both target jade file src, and also the target files
Minor
Update README.md
Update README.md
Update README.md