Replaced "" with static const variable A2STR::NIL.
Replaced string comparison against "" with std::string::empty().
Added PROTO_* to Request class and use them as a protocol string
constant.
Made "started", "stopped", "completed" static const variable in
AnnounceList class.
Move extension from BtRuntime to ExtensionMessageFactory, because
extension can be specified per peer, not per torrent.
* src/DefaultBtInteractive.cc
* src/BtRuntime.h
* src/ExtendedMessagingAware.h
* src/ExtensionMessageFactory.h
* src/DefaultExtensionMessageFactory.cc
* test/DefaultExtensionMessageFactoryTest.cc