Changes license to Apache v2 and adds headers for the core files

The copyright year is fixed to 2013 for all files:

`Copyright 2013 Mozilla Foundation`

The following files were not affected:

examples/*
lib/*
src/avm2/bin/plot/*
src/avm2/compiler/c4/ir.js
src/avm2/compiler/lljs/*
src/avm2/tests/*
src/avm2/json2.js
src/avm2/xregexp.js
src/swf/filters/*
tests/*

**/Makefile
**/*.py