Create CONTRIBUTING.md
First version of CONTRIBUTING.md
Announcer: add >>on: aClass doOnce: aBlock

This addition was proposed by Sebastian Sastre (@sebastianconcept)
on the mailinglist.
Announcer: add first unit tests
Unittests are not an absolute hard requirement
How to build Amber with Grunt
Remove Grunt specific paragraph and redirect to CONTRIBUTING.md
cli: JS fixes
FileServer: enhance warning when no index.html file can be found
FileServer: respond with 'Not Found' instead of 'Internal Server Error'
FileServer: enhance returned error page for 404 response
FileServer: clarify how to use --fallback-page
amberc.js: update documentation
Merge pull request #734 from herby/goodies

A few convenient methods and small cleanup
Merge pull request #733 from mkroehnert/cli

Amber CLI: JavaScript cleanup + enhanced error reporting for 'amber serve'
Merge pull request #732 from mkroehnert/contributing

Add CONTRIBUTING.md
Merge pull request #731 from mkroehnert/announcement

Add Announcement>>on: aClass doOnce: aBlock
tests for if{,Not}Empty:
ifNotEmpty: returns self if empty
combined ifEmpty:ifNotEmpty: and vice versa