Added model validation to DjangoItem.
Added "exclude" parameter testing to unittests.
Merge pull request #218 from Mimino666/django-item-validation

Django item validation
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