Extract alert handling to separate functions.
Replaced dynamic_cast with switch on alert type.

Conflicts:
	src/qtlibtorrent/qbtsession.cpp
Merge pull request #1730 from sorokin/no-dyn-cast

split handling of alerts into several functions and use switch instead of dynamic_casts
Fix HTTP request parsing on Qt5.
Remove old JSON classes.
Add QJson files.
Use new JSON parser/generator.

Qt4: Use QJson (http://qjson.sourceforge.net).
Qt5: Use native Qt JSON API.