Add LESS precompiler support

[thomshouse]

I'm planning on adding LESS precompiler support to Escher using the leafo/lessphp library.

There will be a few components to this feature:

• A less compiler helper.
• A $header->addCompiledLESS('path/to/stylesheet.less'); function that will have branching behavior (see below).
• A subdirectory designated for compiled CSS files to be stored in.

The $header->addCompiledLESS() function will behave as follows:

• If the compiled CSS file does not exist, run $less->compileFile().
• If compiled CSS exists and user guest or non-admin, do nothing.
• If user is logged in as system admin and request header includes no-cache, run $less->compileFile().
• In any event, add URL of compiled CSS to the headers.