Bugfix: reset footnote count after each «text» call

[rubenrubiob]

This addresses #97 : if you are parsing multiple text within a loop, the footnote count is never reset, so it keeps adding values.

For the tests, I added a new method for testing two calls to the sample text containing the footnotes and checking that after both calls the content is the same. I do not like it that much, because it leaves the code for the test a little bit dirty, as it uses the ParsedownTest tests. Another solution would be to create a new test file or, even cleaner, to perform two calls in ParsedownTest. I would change them if it is necessary.