Move stop callback concerns from Engine to Application.
processor.py: Workaround PhantomJS timeouts.

Re: chfoo/wpull#137
Add --phantomjs-exe option.
observer: Allow handlers to be removed during notify.

Re: https://github.com/chfoo/wpull/commit/b12d77dd551c640a1f99c6ca4ac549e16dd6f533#commitcomment-6755015
scraper.py: Add srcset link scrape.

Closes chfoo/wpull#134
changelog: Update latest to 0.36.

[ci skip]
Bump version to 0.37a1.
Merge commit '54e0b9ec47694f641e2d59f2e305c47553346034' into topic/asyncio

Conflicts:
	wpull/phantomjs.py
	wpull/processor.py

[ci skip]
WIP Fix up dns.py.

[ci skip]