Fix for Undefined index: _regular_price, _sale_price, _tax_status, _tax_class, _visibility, _purchase_note

Undefined index appears for: _regular_price, _sale_price, _tax_status,
_tax_class, _visibility, _purchase_note, when fields are empty, for
example.

Notice: Undefined index: _tax_class in
woocommerce\admin\post-types\writepanels\writepanel-product_data.php on
line 678
Fixed my merge gone bad
Readme for cross sells merge fix
Fixed posting shipping method when only one is available

As committed on master branch:
- https://github.com/woothemes/woocommerce/commit/d8ba734d4f71a050df82947d2a8184f7dd8e515d
Fixed query breaking when using some product widgets and closes #3333
Add actions for attribute create/update/delete

As committed on master branch via:
- https://github.com/woothemes/woocommerce/pull/3298
2.0.12 version bump
Changed ISO code for Ceuta (CE) according to http://es.wikipedia.org/wiki/ISO_3166-2:ES
Merge pull request #3364 from samuelaguilera/master

Changed ISO code for Ceuta (CE) according with http://es.wikipedia.org/wiki/ISO_3166-2:ES
Styling tweaks for cart item meta
Enable for all taxonomies
Post types need to exist first
add a class to the variable pricing table row
use switch case for variation bulk edit. add action hook for new cases
Work on charts/reports
Stripslashes
flot min
Correct sanitization of option fields.

@coenjacobs if we do do another maintainence release, this one would be
beneficial
Update add-to-cart-variation.js
WooCommerce fails to Activate
declared $helpers as a blank array
(to keep things tidy) removed $this->helpers from init().