Add new cache config options

The new options are `CSSC_AUTOCLEAR` and `CSSC_MAX_AGE`. Those settings
will be used by the cache auto-clearing feature.
Move tester classes to Tester namespace
Rename CSSc\CSSc class to CSSc\Core
Implement a better way of checking argument types
Remove docs folder

The docs should be included in a website dedicated to CSScompiler in a
future date.

Often the docs are outdated in each commit, which is a pretty bad thing.
Rename some classes
Update preprocessor submodules
Rename class CSSc\Parser\Tokens to CSSc\Parser\Tokenizer
Add class CSSc\Parser\ImportSolver