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
Adding translatable page title to download file error page
CZ lang completed

CZ lang completely translated.
Added needed states for Spain
Update woocommerce-es_ES.po

Updated untranslated strings, countries and currencies, and fuzzy translations.
Merge pull request #3345 from jbermudez/patch-1

Update woocommerce-es_ES.po
Merge pull request #3349 from MarekRabas/master

cz lang completed
Merge pull request #3344 from samuelaguilera/master

Added needed states for Spain
Merge pull request #3338 from BFTrick/master

Add Translatable Page Title to Download File Error Page
Futher standardisation of cart and order loops. Closes #3348.
readme
Is checkout pay page function
Trigger an action when the variation form is attached
dummy data csvs. closes #3119
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