Move commands to the concert script

It does not make much sense to export the functions as part of the API, hence
everything is moved to the script itself. To allow easier extension later on
every command as been turned into an object.
Move exit_if_not_exists to concert.session
Integration of Spyder as an external command
Use helper function instead of handcrafting path
Move Bunch class to concert.helpers
Delay importing urllib2
Split session module into utils and management

Warning: this breaks the ddoc, dstate etc. imports!