API: Add sync_blacklisted_patterns for configuring files to skip in sync tasks

Fixes http://open.silverstripe.org/ticket/6210.

Replaces the hardcoded file patterns from Folder::syncChildren() with a new static Filesystem::$sync_blacklisted_patterns to describe files and folder names to skip when running Folder::sync().

Added unit test for Folder::sync()

Extended Folder::sync() to report on the number of file / folders skipped.
Merge pull request #1874 from wilr/open6210

API: Add sync_blacklisted_patterns for configuring files to skip in sync()