Drop libxml2 selectors backend
working implementaion of unified api
Remove CSS*Selector classes and port its tests
port scrapy shell to unified selector api
default xpath selector was html
port tests to new Selector class
remove internal references to old selector classes and api
Updates docs to reflect unified selectors api
remove more references to libxml2
update release notes
Parameters to Request() in wrong order

Implied that callback wasn't the first optional unnamed parameter.
Move callback blob; mention errback
fix typos and wording on selector's introduction
use `sel` name for Selector's instances in docs, internals and shell
Replace `contenttype` references by `type`

The type to choose from is the selector type, not the input type. A
content-type doesn't make sense in this context.
Merge pull request #426 from scrapy/selectors-unified

[MRG] Selectors unified API
Merge pull request #226 from scraperdragon/patch-1

Parameters to Request() in wrong order
typo fix in TextResponse docs
fix references to old selector naming in docs
update requirement for cssselect >= 0.9