Fixed typo

'persitent' --> 'persistent'
Merge pull request #215 from kuyan/patch-1

Fixed typo
Added --spider option to "shell" command.
lint and improve images pipeline error logging
Merge pull request #216 from Mimino666/shell-spider-option

Added --spider option to "shell" command.
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