Fixed KeyRelease events sometimes not reported on Linux (#404)

Conflicts:
	src/sfml/window/Linux/WindowImplX11.cpp
	src/sfml/window/Linux/WindowImplX11.hpp
Fixed windows bigger than the desktop not appearing on Windows (#215)
Solved graphics resources not updated or corrupted when loaded in a thread (#411)

Conflicts:
	src/sfml/graphics/Font.cpp
Fixed ContextSettings ignored on Linux when creating a window (#35)

Conflicts:
	src/sfml/window/Linux/GlxContext.cpp
	src/sfml/window/Linux/WindowImplX11.cpp
	src/sfml/window/Linux/WindowImplX11.hpp
	src/sfml/window/OSX/WindowImplCocoa.mm
	src/sfml/window/Win32/WindowImplWin32.cpp
	src/sfml/window/Win32/WindowImplWin32.hpp
	src/sfml/window/WindowImpl.cpp
	src/sfml/window/WindowImpl.hpp
Checking errors in RenderTarget::pushGLStates() to avoid generating false error messages when user leaves unchecked OpenGL errors (#340)

Conflicts:
	src/sfml/graphics/RenderTarget.cpp
Added .travis.yml
Fixed ContextSettings ignored when creating a 3.x/4.x OpenGL context on Linux (#258)

Conflicts:
	src/sfml/window/Linux/GlxContext.cpp
Added a workaround for a bug in the OS X implementation of OpenAL (unsupported channel count no properly detected) (#201)
Fixed bounding rect of sf::Text ignoring whitespaces (216)
Fixed MouseMove event sometimes not generated when holding left button on Windows (#225)