Use Erubis when available for faster startup

Haml currently compiles 128 ERB templates during bootup as a result of `#def_static_method`. This takes roughly half a second on my machine.

By using Erubis' TinyRuby rather than ERB to do the parsing, this takes about 1/3rd as much time. If Erubis is not available, Haml will fall back to ERB.