code: public static Validator<CharSequence> letter(@NonNull final Context context,
                                                   @NonNull final Case caseSensitivity,
                                                   final boolean allowSpaces,
                                                   @NonNull final char... allowedCharacters) {
          return new LetterValidator(context, R.string.default_error_message, caseSensitivity,
                  allowSpaces, allowedCharacters);
      }
docs: Creates and returns a validator, which allows to validate texts to ensure, that they only
  contain letters. Letters are considered to be all alphabetical characters from A to B. It is
  possible to specify, whether only uppercase or lowercase letters should be accepted, or if
  they should be threatened case-insensitive. Additionally, it is possible to specify whether
  spaces should be allowed and to add special characters, which should also be accepted. Empty
  texts are also accepted.
  
  @param context
  The context, which should be used to retrieve the error message, as an instance of
  the class {@link Context}. The context may not be null
  @param caseSensitivity
  The case senstivitiy, which should be used by the validator, as a value of the enum
  {@link Case}. The value may either be <code>UPPERCASE</code>, <code>LOWERCASE</code>
  or <code>CASE_INSENSITIVE</code>
  @param allowSpaces
  True, if spaces should be allowed, false otherwise
  @param allowedCharacters
  The allowed special characters as an array of the type <code>char</code>. The array
  may not be null
  @return The validator, which has been created, as an instance of the type {@link Validator}