Warnings on Log. Deprecated Libraries

[RenanGBarreto]

Few warning msgs are shown on the output log because the project users deprecated libraries.

Sample:

2016-04-06 07:15:15 [py.warnings] WARNING: c:\python27\lib\site-packages\scrapy_mongodb.py:30: ScrapyDeprecationWarning: Module `scrapy.log` has been deprecated, Scrapy now relies on the builtin Python library for logging. Read the updated logging entry in the documentation to learn more.  from scrapy import log

2016-04-06 07:15:15 [py.warnings] WARNING: c:\python27\lib\site-packages\scrapy_mongodb.py:31: ScrapyDeprecationWarning: Module `scrapy.contrib.exporter` is deprecated, use `scrapy.exporters` instead  from scrapy.contrib.exporter import BaseItemExporter

2016-04-06 07:15:15 [py.warnings] WARNING: c:\python27\lib\site-packages\scrapy_mongodb.py:105: ScrapyDeprecationWarning: log.msg has been deprecated, create a python logger and log through it instead  self.config['collection']))

2016-04-06 07:17:44 [py.warnings] WARNING: c:\python27\lib\site-packages\scrapy_mongodb.py:256: ScrapyDeprecationWarning: log.msg has been deprecated, create a python logger and log through it instead  spider=spider)

This pull request may solve the issue but need to be reviewed: Pull Request 31