Implementig and decoupling advanced cache key generation; BC break.

Repurposing `Cache::key()`. It now deals with advanced cache key generation
and also makes keys safe to use with given adapter configuration.

BC break: Method signature of `Cache::key()` changed. This method
was probably seldomly used because of its very limited featureset.

BC break of undocumented feature: key generator functions
cannot be passed to `Cache::{read,write,delete,increment,decrement}()`
anymore. See explanation below.

- Method signature of `Cache::key()` changed. The first
  parameter is now - as in other cache methods - the name
  of the cache configuration to use.

  ```
  Cache::key('post', $data); // old
  Cache::key('default', 'post', $data); // new
  ```

- Decoupling cache key generation.

  Advanced key generation has been moved out of
  `Cache::{read,write,delete,increment,decrement}()`. Key generator
  functions may not be passed to these methods anymore. Instead generate
  the key beforehand using the new `Cache::key()`.

  Key generator functions for read and delete needed to be implemented
  in a different way then those for i.e. write as data could not be
  passed to them in any case.

  ```
  $data = array('id' => 23);

  // old: removed
  Cache::write('default', function($data) { // ... }, $data);

  // new: 2 step process
  $key = Cache::key('default', function($data) { // ... }, $data);
  Cache::write('default', $key, $data);
  ```

- The method can now be used to generate safe cache keys.

- The method can now be used to suffix cache keys with
  hashes of data.

- Adding `keys()` method for adapters.

- Adding tests.

- Adding documentation.