include Poco/URI.h for all platforms

Previously was only included on Linux and Windows, preventing compilation on OS X
as drop_cb made use of it
add .gitignore

default of ignore, also ignores xml settings folder
move multi window code to examples in xcode

previously were listed as being in src and couldn't be found
change ofxAppGLFWWindowMulti.cpp to .mm

as it needs to be treated as objective-c++ this doesn't require changing settings in Xcode
Merge pull request #4 from acarabott/0.8.2

include Poco/URI.h for all platforms