Turn off error reporting during an export

We need to turn off error reporting while we're building an export so that PHP notices and other errors don't break an export.
Add check for IS_WPCOM as a forbidden constant.
Improve regex for matching constants.

The regex now also detects constants with a single or double quotes
before them. This catches constants being used in is_defined().
Remove unused BadThingsCheck
Add WidgetCheck

Checks for proper widget and sidebar usage.
Remove unused checks from WP.com Review type.
Add a ZipScanner, which scans zip files without extraction

This won't support any checks that assume the path lives on a
real filesystem, e.g. phpcs & clamav
Return if we can't open the zip
Use $zip->getFromIndex
Merge pull request #128 from ehg/zip-scanner

Add a ZipScanner, which scans zip files without extraction