Add disk cache.

[soncodi]

Add simple serialized flat-file cache to store cache files.

[everplays]

thanks @ruff. would you please make the following changes?

• indentation from 2 spaces to 4 spaces.
• I think it'll be good idea to make mkdir create directories recursively.
• remove @ operator, specifically in get / set / remove methods, people should get notices if things are going wrong.

[soncodi]

hello @everplays:

• indent is 4 spaces - which lines are incorrect?
• the third param to mkdir (true) does this, per http://php.net/manual/en/function.mkdir.php
• removed @ from all fs calls other than in the ctor, where there is a test for the result

[everplays]

@ruff sorry, my bad. quick look caused that I miss that arguments of mkdir are already there. same about indentation. thanks :)

[soncodi]

@everplays thanks!