Add performance logging for build

[LightGuard]

Currently this will add execution time for the various steps. I'll create a new option (not sure what it will be right now) that will enable the performance logging. Logging will then happen in the .awestruct/perf.log file

Execution time will be broken up into

• Overall time
• Each step in engine.rb
• Each extension
• Each transformer
• Each page, possibly also tied to the verbose flag as this can really slow down a site if there are many pages being built