Allow scheme-relative urls

URLs starting with "//" will choose the scheme of the current page.  This allows more flexibility when loading content from CDNs.
Don't rewrite scheme-relative URLs
Update contributors.md per policy
Optimization for LiftSession.findSnippetClass

snippet name to class map will be cached on the LiftSession object to
avoid duplicate searches. This is to a avoid a performance problem where
the searches become expensive as many snippet packages are added and the
number of classes loaded is very large.
optimized version of compact(render(json)) using a StringBuilder

For large JValue objects, the `compact(render(value))` can be very CPU
intensive. This adds a `compactRender` convenience method that renders
a JValue directly to a String without the intermediate Document. A
StringBuilder is used to avoid String concatenations.

I added a new benchmark comparing the two serialization operations.
Results on the large JValue running on core i7 MBP:
lift-json (ser compact(render(jvalue))	45607ms
lift-json (ser compactRender(jvalue)	35144ms

So the performance improvement can be quite large.
Implement SelectableOption, and use it in place of tuples for select*.
Add an implicit conversion from tuple to SelectableOption.
Refactor, improve readability for SHtml.checkbox[T].
Readability improvements to SHtml.secureOptions[T].
Implement and use SelectableOptionWithNonce during processing.

The SelectableOptionWithNonce class serves as a helper for keeping track
of a SelectableOption's attrs during processing.
Readability improvements to SHtml.secureMultiOptions[T].
Use SelectableOptionWithNonce in secureMultiOptions[T].
Set custom attributes for option tags in select_*.
Fix a MatchError related to the new SelectableOption.
Populate custom option attributes in untrustedSelect_*.
Populate custom option attributes in untrustedMultiSelect_*.
Populate custom option attributes in ajaxUntrustedSelect_*.
Implement SelectableOption support for ajaxSelect*
Make the type bound on SelectableOption* more tolerant.
Clean up some rouge cases.