Add support for recursive CTEs

[arikkfir]

This change adds support for recursive CTEs by introducing the Recursive() method to CTETableBuilder. This method is automatically called by the existing CTETable(name, cols) function, but can also be called manually by calling it on a pre-existing CTETableBuilder instance.

It also adds an example for recursive CTEs.

Fixes #161.

[coveralls]

coverage: 96.451% (+0.01%) from 96.441% when pulling 4b034a7bb7d8d81d7b5c1cb0d2f67803ede27f07 on arikkfir:add-recursive-cte-support into 2cc1f8cad6256b0e8271f3a213256b4a8b2a4fb3 on huandu:master.

[huandu]

LGTM. Thanks for your contribution!