Properly fix missing template names in backtrace.

The tilt signature only allows template content *or* a template
filename, not both. As a result, if you read the contents of a template
before it is passed to tilt (e.g. to get the metadata out) and there's
an error in the template it will only show up as '(__TEMPLATE__)' in the
backtrace (which is confusing and unnecessary).

With this patch the signature of the tilt method is changed to include
the content and/or the filename, allowing Tilt to include the filname in
a possible backtrace.

Without patch:

 - Creating 'output'
(__TEMPLATE__):5:in `evaluate_source': undefined local variable or method `example' for #<Object:0x8bd8410> (NameError)
  from /var/lib/gems/1.9.1/gems/tilt-0.9/lib/tilt.rb:254:in `instance_eval'
...

With patch:

 - Creating 'output'
/tmp/frank/dynamic/_top.haml:5:in `evaluate_source': undefined local variable or method `example' for #<Object:0x9564470> (NameError)
  from /var/lib/gems/1.9.1/gems/tilt-0.9/lib/tilt.rb:254:in `instance_eval'
...

lib/frank/base.rb

@@ -105,14 +105,14 @@ def render(path, partial = false, local_vars = nil)
       # let tilt determine the template handler
       # and return some template markup
       if layout.nil?
-        tilt(page, ext, template, locals)
+        tilt(page, ext, template, template_path, locals)
       else
         layout_path = File.join(Frank.root, Frank.layouts_folder, layout)
         # add layout_path to locals
         raise Frank::TemplateError, "Layout not found #{layout_path}" unless File.exist? layout_path
 
-        page_content = tilt(page, ext, template, locals)
-        tilt(page, File.extname(layout), layout_path, locals) do
+        page_content = tilt(page, ext, template, template_path, locals)
+        tilt(page, File.extname(layout), nil, layout_path, locals) do
           page_content
         end
       end
@@ -184,14 +184,9 @@ def layout_ext_or_first(layout_exts, ext)
     end
 
     # render a page using tilt and get the result template markup back
-    def tilt(page, ext, source, locals={}, &block)
-      Tilt[ext].new do
-        source = source.to_str if source.respond_to?(:to_str)
-        if source.match(/^[^\n]+$/) && File.exist?(source)
-          File.read(source)
-        else
-          source
-        end
+    def tilt(page, ext, source, filename, locals={}, &block)
+      Tilt[ext].new(filename) do
+        source || File.read(filename)
       end.render(page, locals=locals, &block)
     end