feat(storageKey): add a storageKey method

The storageKey method allows the user to change a key, which will be used
to store the language code. So, the user can change this key during the
configuration phase now.

BREAKING CHANGE: The $STORAGE_KEY isn't represent a current storage key
from now. To discover which key is used now you have to call the storageKey
method without params.
test(storageKey): add tests for a storageKey method
chore(storageKey): add a basic comment docs
chore(preferredLanguage): fix tests (restructure, delete)

* change the structure of the preferredLanguage tests to be more logical
* delete some tests in order to DRY
Merge pull request #39 from DWand/storage-key-feat

storageKey feature + preferredLanguage tests fix