Implement caching for on-demand compiling with plessc

[thybzi]

Implemented caching feature (that exists in main module) to plessc command-line tool.
Added -q key to plessc in order to enable this new feature.

The code if based on this example from lessphp documentation:
http://leafo.net/lessphp/docs/#compiling_automatically

Generates .lessphp files beside corresponded less-files compiled, and then tries to read them and use as cache.
Rewrites cache files when needed ($cache_new["updated"] > $cache["updated"]).

[thybzi]

This thing about caching is what we are using intensively in a project with lot of LESS styles compiled with lessphp.

Please accept this request, or explain problems/issues it can cause (though I can't imagine any - because it is a new feature, changing nothing in existing functional :)

I suppose this feature could be useful for everyone who uses lessphp.

[thybzi]

Please merge this pull request, I think this feature will be useful for everyone using plessc CLI tool. Also, it shouldn't break anything currently working.

[thybzi]

Would you please (review and) merge that?

[thybzi]

Why don't accept this cool pull request?