allow non-bundled tilt to be loaded

Tries to require 'tilt' before falling back on the
bundled version - 'sinatra/tilt'. This should result
in any tilt on the $LOAD_PATH overriding the bundled
version.

lib/sinatra/base.rb

@@ -3,9 +3,15 @@ require 'time'
 require 'uri'
 require 'rack'
 require 'rack/builder'
-require 'tilt'
 require 'sinatra/showexceptions'
 
+# require tilt if available; fall back on bundled version.
+begin
+  require 'tilt'
+rescue LoadError
+  require 'sinatra/tilt'
+end
+
 module Sinatra
   VERSION = '0.10.1'