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
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
flot min
WooCommerce fails to Activate
declared $helpers as a blank array
(to keep things tidy) removed $this->helpers from init().
add admin translation
edit german formal language
More charts work
Reports + sparklines
Better averages, and fixed stats for the day
class based reports - top sellers and earners, added cache based on query hash
Sales by product
By cat
by category
Merge pull request #3390 from swicks/master

Fails to activate
Merge pull request #3395 from CoachBirgit/master

edit german formal and add admin language files
Merge pull request #3406 from kloon/master

Extension changelog version info checking
Merge pull request #3385 from helgatheviking/feature/variable-bulk-edit-js

use switch case for variation bulk edit. add action hook for new cases (for #3383)
Merge pull request #3372 from helgatheviking/feature/variable-pricing-meta-tr-class

add a class to the variable pricing table row