Merge pull request #7 from simpleinvoices/master

Snch with Upline Master
Replace all $LANG['Custom_Fields'] => $LANG['custom_fields'] and all $LANG.Custom_Fields => $Lang.custom_fields for uniformity
All strings entered for lang/da_DK/lang.php - Dansk (Danish)
Merge pull request #89 from apmuthu/master

Lang consistency case sensivity fixed, da_DK strings entered