Pass into the model only existing fields

Model fields has default values and when field doesn't exists in item container, added `None` instead default value. My patch solves this problem.
Simple test for default values of model in DjangoItem
append parse command to example code sections in docs. closes #162
Recommend pypi as single way to install on Windows
added ppt to scrapy.linkextractor.IGNORED_EXTENSIONS
added more extensions to ignore on link extractors
Merge pull request #160 from midiotthimble/patch-1

Pass into the model only existing fields

test passing OK and changes makes sense
Small improvement on *MemoryQueue performance (mainly on push time)
Merge pull request #165 from andrix/master

Just a simple refactoring on *MemoryQueue classes that improve the performance
SEP-017 contracts: first draft
Removed per-spider settings concept, and scrapy.conf.settings singleton from many extensions and middlewares. There are some still remaining, that will be removed in future commits
Removed signals/stats singletons

This change removes singletons for stats collection and signal
dispatching facilities, by making them a member of the Crawler class.

Here are some examples to illustrates the old and new API:

Signals - before:

    from scrapy import signals
    from scrapy.xlib.pydispatch import dispatcher
    dispatcher.connect(self.spider_opened, signals.spider_opened)

Signals - now:

    from scrapy import signals
    crawler.signals.connect(self.spider.opened, signals.spider_opened)

Stats collection - before:

    from scrapy.stats import stats
    stats.inc_value('foo')

Stats collection - now:

    crawler.stats.inc_value('foo')

Backwards compatibility was retained as much as possible and the old API
has been properly flagged with deprecation warnings.
use ScrapyJSONEncoder JsonItemExporter & JsonLinesItemExporter, to support nested items properly
fixed typo in tests
SEP-017 contracts

* load contracts from settings
* refactored contracts manager
* fixed callback bug, which caused responses to be evaluated with a
wrong callback sometimes
* "returns" contract
Updated documentation after singleton removal changes.

Also removed some unused code and made some minor additional
refactoring.
Merge branch 'singleton_removal'
fixed bug in FeedExports extension, introduced in previous commit
promoted DjangoItem to main contrib
added file missed in previous commit