Cache made by var_export (performances)

Added:
  - choice of hash algo (for large sets of cache)
  - cache made by var_export instead of json_encode
  - fs cache is now put in a php file to be opcode cached by APC (PHP<5.5) or OPcache (PHP>=5.5)

var_export conv : if your cache values are only numbers, var_export will create larger files. But in this project, cache are mostly characters. So, your files will be smaller because they won't be 'json encoded'. 
- No json_encode/decode : better performances

Other things : var_export returns the exact PHP code so that it could be cached as opcode.