TextSecure: NL translation

ps1: remove unused strings

ps2: added strings to be translated

ps3: added new translations, not complete yet

ps4: suggestions, still not completed

ps5: suggestions, everything is now translated

ps6/7/8: suggestions

ps9: fix copyright

ps10: fix typo

Change-Id: Ia56e6d3c12c12b3c94608673d730adaa9da86443