po filles generation.
All the quotation marks must be screened by adding a backslash before them. Otherwise we cannot generate .mo file.
Fix
Merge pull request #1978 from marsuboss/patch-1

Fix translation french
code style fixed
Update upgrade-from-v1.md

Text amended as "A new method" was referring to two. CHanged the text to "New methods".
Merge pull request #1980 from surek/patch-1

Update upgrade-from-v1.md
po now supports obsolete messages
fixes for previous solution
Mark event as handled when filter decides to abort action

fixes #1984
Merge pull request #1986 from cebe/1984-abort-action-event

Mark event as handled when filter decides to abort action
Fixes #1959: `Html::activeCheckbox` wasn't respecting custom values for checked/unchecked state
better fix for #1959.
Merge pull request #1979 from SergeiKutanov/po_generation

po filles generation.
minor formatting fixes
Missing "md" extension added for "Using 3rd-Party Libraries" guide part
Merge branch 'master' of github.com:yiisoft/yii2 into 1837-3rd-party-doc