More thoroughly test method generate_html

Create a battery of three tests for method `generate_html`. These tests check
that:

* The string returned is unicode.
* If the `length` argument is omitted, the contents of the returned HTML tag are
  at least one character long.
* If the `length` argument is provided, the contents of the returned HTML tag
  are of the provided length.

Fixes #33.

Add Kedar Bidarkar to the authors file.