Short term:

- add repository dependencies and conflicts metadata

- entropy.client.interfaces code refactoring
- wrap available repositories usage around a ResourceLock, this will
  decrease the need for an exclusively held Entropy Resources Lock.

- conditional deps:
  enabled with ETP_PORTAGE_CONDITIONAL_DEPS_ENABLE
  Test package generation and entropy client behaviour
  (both direct and inverse dependencies calculation)


 Long term:

 - move variables away from etpConst
   (to SystemSettings || Client || Server as static methods)
 - Python 3.3+ support

 - [2014] remove repositories parsing from repositories.conf
          also see _conf_add_repository(), _conf_remove_repository(),
          shift_repository()
 - [2015] remove non-ini repositories parsing support from repositories.conf.*
 - [2015] remove repositories parsing from server.conf
