Add ability to define some cosntants in WP_CONFIG if desired
Detect cache enabled from WordPRess settings.
Allow cache config to be set in WP_CONFIG.PHP
Cart Widget will now respect cache detection setting.
Check using $cache_enabled setting removed becuase that setting can't be found in WordPress documentation.
WP_CACHE defined constant user per documentation here http://codex.wordpress.org/Editing_wp-config.php#Cache
meta key translation for future upgrade implementation begins
Changed constant to WPSC_PAGE_CACHE_IN_USE from WPSC_DONT_CACHE for clarity
added inline comment about caching constant
Merge remote-tracking branch 'upstream/master' into wp-cache-detection

Conflicts:
	wpsc-includes/wpsc-meta-visitor.php
Merge pull request #908 from JeffPyeBrook/wp-cache-detection

Control cached cart widget based on detecting cache enabled from WordPress settings