Refactor and fix class method `generate_cjk`

Refactor method with the intent of making it easier to understand. The method is
now shorter (6 LoC instead of 12) and has more descriptive variable names.

Also fix a minor bug. Formerly, the method would never return the character with
codepoint 0x9FCC, which is at the upper limit of the CJK character set.
Refactor method

Use `random.randint` instead of `random.randrange`. The former returns a number
N where `a <= N <= b`.
Add `generate_utf8` and update `generate_string`

Add method `generate_utf8`, which returns a string composed randomly from the
entire set of UTF-8 characters. Update method `generate_string` so that it calls
`generate_utf8` and `generate_cjk` when asked for a "utf8" or "cjk" string,
respectively.

Update docstring for method `generate_string` to reflect these changes.
Add tests for method `generate_utf8`
Merge pull request #20 from Ichimonji10/master

Refactor and fix class method `generate_cjk`