Test, developers-only, version finished

Code requires a lot of refactoring.
Updated Core
Fixed Travis
Inserted a screenshot
Added Travis build status image
Improved core.cpp

Now it uses friend ids whenever it's possible.
Some small corrections
Deleted Core
Added Core as a git submodule
Modified project file to work with the submodule
Updated Core submodule
Separated Core's sources
Updated Core submodule
Removed random whitespaces
Removed Username fiels in AddFriendDialog

Because I don't use that value anymore and Core will be exchanging usernames
at some point, so there won't be any point in specifying it by hand.
Fixed AddFriendDialog size
Implemented settings, improved DHT dialog
Updated Core submodule
Fixed a bug in settings
Use selected DHT server in Core