add InitMessage() to noui and use debug.log for GUI

- this pull adds an InitMessage() function to noui.cpp, which outputs init
  messages to debug.log (this allows to remove some printf() calls from
  init.cpp)
- change InitMessage() in bitcoin.cpp to also write init messages to
  debug.log to ensure nothting is missing in the log because of the
  removal of printf() calls in init.cpp
Merge pull request #2171 from Diapolo/init

add InitMessage() to noui and use debug.log for GUI