Update document formatter for ruby 1.9
Fix the document of draw_line (thanks a  report from PJ L
Update Thanks in README
Fix a bug of SDL::TTF#draw* when empty string is given

These methods raise error when empty string is given
as a rendered string, but these methods should do nothing in that case.
  * http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=583274
  * http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=714501
Fix a compilation problem on FreeBSD.

In FreeBSD, ruby defines a C macro replacing "close" identifier.
This causes a compile error of rubysdl_rwops.c and this change
suppresses the macro.

Thanks for the report by Rusmir Dusko.