Refactor assert_select

- Use implicit method level rescue
- rm root variable
Use implicit begin scope in nest_selection.
Format some long lines to 80 characters.
Use implicit method level rescue in css_select.
Extract count_description to a concern.
Reshape relationship between selector_assertions and HTMLSelector

- move root finding to HTMLSelector
- rename, refactor and privatize HTMLSelector methods
- turn context into a class variable
- turn accessors into readers