translation update 2013-06-25

- updates bitcoinstrings.cpp and bitcoin_en.ts so new translations can be
  made on Transifex
Added comments to sync.h to make it easier to understand the macros
Merge pull request #2174 from CodeShark/sync_macro_clarification

Added comments to sync.h to make it easier to understand the macros
Merge pull request #2793 from Diapolo/translations

translation update 2013-06-25