Add Soup.is_document and Soup.is_text to the public API

[dmbaturin]

For code that manipulates element trees, it's handy to be able to check what kind of a node a function is dealing with.

As of 1.0.0, only Soup.is_element is exposed in the public API. Soup.is_document exists internally but isn't exposed, and Soup.is_text doesn't exist at all. All three are trivial to add and expose.

This PR exposes a complete set of node type checks.

If you want me to add tests for those functions, I can do it as well.

[dmbaturin]

Do you see any possible issues with the PR?

[aantron]

Thank you!