Added Contextual Help for Time Passes

Added contextual help for time passes and according translations.
Updated Readme

Updated readme.txt to cover time passes and vouchers, shortcode improvements, bulk price editor and content rating.
Updated Readme

Removed the note that it would be possible to create vouchers which reduce a price by a certain percentage. Currently, it is only possible to set a specific price for a voucher.
Update README.md
Merge branch 'master' into feature/196-Add-time-limited-passes

Conflicts:
	laterpay/languages/laterpay-de_DE.mo
	laterpay/languages/laterpay-de_DE.po