Adopt rake-compiler directory layout, remove (hopefully obsolete) source files

Conflicts:
	GosuImpl/TextInputX.cpp
Misc lib/gosu.rb & extconf.rb cleanup

Conflicts:
	ext/gosu/extconf.rb
Update .gitignore
Fix Gosu.podspec (OS X only, iOS still FUBAR)

Conflicts:
	examples/Tutorial/Tutorial.xcodeproj/project.pbxproj
Fix Raspbian detection for older installations