Make sure grouped term exists before using it as an object.

Fixes Trying to get property of non-object in… PHP notice.
More PHP notice/warnings data validation

Fixes PHP Notice: Undefined offset: X
Fix styling issue for add-ons list

Min-height wasn't enough for some extension descriptions. Seen here: http://cld.wthms.co/7ypt
#3921
Use subtotal for free shipping, remove unused min_amount from abstract #3924
Fix is_order_received_page()
Merge pull request #3929 from leewillis77/is_order_received_page_fix

Fix is_order_received_page()
Code from .17 + readme
Merge pull request #3923 from ragulka/estonian-translation

Update Estonian translation
Merge pull request #3918 from scottbasgaard/master

Make sure grouped term exists before using it as an object.
woocommerce_before_single_variation and after. Removed un-needed div. Closes #3906
Check product var is set
Spelling corrections in readme.txt
Minify
Merge remote-tracking branch 'woo/master'
Merge pull request #3932 from tivnet/master

Spelling corrections in readme.txt
get_product_price() : PHPDoc fix
needs_payment for 2.1 #3928
Merge pull request #3935 from tivnet/master

get_product_price() : PHPDoc fix
get_product_subtotal() PHPDoc fix