Case-sensitive lookup (#6) and ST2 compatibility

If no case-insensitive OS (Windows, OSX) is detected, try several
common spellings to find readme files.

If sublime.find_resources is not defined (as in Sublime 2), use a
fallback that mimicks the original behavior to the extent needed:
look for files matching a pattern inside subdirectories of
sublime.packages_path(). Unlike the original function, lookup is
NOT recursive and does NOT include sublime.installed_packages_path().