implemented precondition os based strategy, implemented brew being ma…

[EugenMayer]

implemented precondition os based strategy, implemented brew being ma…ndatory, prepared 0.3.0 release, fixed upgrade check being run on first installation. Install osx libs using brew only

@ignatiusreza have a look at this PR, its is most probably gearing towards what you do or started.

i implemented this since i want to make brew mandatory on OSX, this would have broken Linux support forever, so i implemented it the proper way as we planned.

Linux is yet stubbed as you see - you dare tackling this? :)

[ignatiusreza]

@EugenMayer WOW! That's quite some changes! I'll see what I can do on the Linux branch..

before that though.. let's clarify on when we want to on support for linux.. in #265 we agreed that unison/rsync shouldn't be needed in linux, and instead we could introduce a new sync strategy using native docker volume create.. after that, we can update this normalization steps into something like:

def sync_strategy_for(sync_config)
  sync_strategy = sync_config['sync_strategy']

  if ['rsync', 'unison', 'volume'].include?(sync_strategy)
    sync_strategy
  else 
    default_sync_strategy
  end
end

def watch_strategy_for(sync_config)
  watch_strategy = sync_config['watch_strategy']
  watch_strategy = 'dummy' if watch_strategy == 'disable'

  if ['fswatch', 'unison', 'dummy'].include?(watch_strategy)
    watch_strategy
  else
    default_watch_strategy
  end
end

def default_sync_strategy
  case true
  when OS.linux? then 'volume'
  else 'unison'
  end
end

def default_watch_strategy
  case sync_strategy_for(sync_config)
  when 'rsync' then 'fswatch'
  when 'unison' then 'unison'
  when 'volume' then 'dummy'
  else raise "you shouldn't be here"
  end
end

which would be suffice in making sure that docker-sync is usable in linux.. we can also add some check under preconditions_linux that the necessary binary is available if user are so inclined on using rsync or unison, but I'm not so sure in auto installing unison binary.. wdyt?