[MRG+1] Py3 S3 botocore

More liberal Content-Disposition header parsing

Fixes #1782

botocore not only does not allow passing our own Date header, but does not handle x-amz-date according to the spec

[MRG+1] Add fallback and deprecation warning for pydispatch (fixes #1762)

[MRG+1] PY3: Fix TypeError when outputting to stdout

[MRG+1] PY3: Implement some attributes of WrappedRequest required in Python 3

[MRG+1] PY3: Fix SitemapSpider to extract sitemap urls from robots.txt properly

This will fix #1770.

This will fix #1768.

This will fix #1766.

[MRG+1] more efficient ExecutionEngine.spider_is_idle

[MRG+1] MailSender.send: allow passing a charset.

Resolves Issue #348