docs: list lxml as installation prerequisite
Adds HtmlCSSSelector and XmlCSSSelector classes, cssselect as optional dependency.

Ported .get() from _Element and .text_content() from HTMLMixin

Add CSS selectors to scrapy shell

Documenting CSS Selectors: Constructing selectors

Documenting CSS Selectors: Using Selectors

Make CSS Selectors a default feature.

Adds XPath powers to CSS Selectors and some syntactic sugar.

Removes methods copied over from lxml.html.HtmlMixin.

Updating docs to use new CSS Selector super powers.

Documenting CSS Selectors: Regular Expressions

Moving section after Nesting section, since it mentions it.

Documenting CSS Selectors: Nesting Selectors

Fix XPath specificity in lxml.selector.CSSSelectorMixin.text

Cleaning up unused stuff from cssel.py

Changing the behavior of lxml.selector.CSSSelectorMixin.text.

Concatenating all of the descendant text nodes is more useful
than returning it in pieces (there's xpath() if you need that).

Documenting CSS Selectors: CSS Selector objects

Documenting CSS Selectors: CSSSelectorList objects

Documenting CSS Selectors: HtmlCSSSelector objects

Documenting CSS Selectors: XmlCSSSelector objects

Fixing some documentations typos and errors

Enforcing the 80-char width lines

Tidying up CSS selectors and CSSSelectorMixin objects

Adding some missing references in documentation.

Fixing lxml.selector.CSSSelectorList.text
rename XPathSelectorList as SelectorList #176
extend css selectors with ":text" and :attribute(<name>) #176
Adapt to latest cssselect API supporting pseudo-elements
Support only ::text and ::attr(name) + add more CSS selector tests
wip