Properly check for a selected shipping method and shipping option at checkout
* Make sure method and option are set and valid before validating checkout
* Make sure validity flag is initialized before testing it
* Test for any shipping modules needing zip code rather than just a shipping module names ups
* Don't clear cart contents on shipping quote selection update after some cart variables have been changed.  It causes changes to be lost, and the current shipping module selection to be lost when the cart page is redisplayed
* Wrap validation functions in cart class methods rather than reach into the cart class and test properties whose allowable values may not be well understood by outside classes
* Add wrapper functions for testing if shipping is enabled
* remove test on non-existent cart item class property

Addresses issue #1314

Thanks to @misulicus for helping isolate this

Conflicts:
	wpsc-core/wpsc-functions.php
removed comments
fixed typo
updated phpDoc
options variable no longer used.
Merge #1318
Save meta values when checkout form submits
Confirms that the most recent meta values are saved when the checkout form is submitted.  The checkout values are restored into customer meta.  THis should be a very inexpensive safeguard against race conditions as the WOrdPress meta infrastructure checks values as they are stored to be sure they are changed.  Unchanged value store attempts noop very quickly
Merge branch 'branch-3.8.14.1' into branch-3.8.14.1-dev-merged

Conflicts:
	wpsc-components/theme-engine-v1/helpers/ajax.php
	wpsc-includes/cart.class.php
	wpsc-includes/wpsc-meta-util.php