DownloaderMiddleware docs: Update process_request, proper explanation of IgnoreRequest.

Also:
* Change terminology to eliminate uses of terms such as "request middleware" to refer to the process_request methods of installed middleware.
* Remove description of "immediate redirection", as it is misleading.

Further changes.
replaced __import__ by importlib.import_module.

Since python 2.7, importlib.import_module is the recommended way to
import modules programmatically.

From __import__'s doc:

    Import a module. Because this function is meant for use by the
Python
    interpreter and not for general use it is better to use
    importlib.import_module() to programmatically import a module.
Replaced remaning __import__(module) calls.

This commit replaces the statements __import__(module) as the previous
replaced the statements __import__(module, {}, {}, ['']).

At first I thought leaving the single-argument calls, but perhaps it's
better to be strict rather than having exceptions to the rule in this
case.
Merge branch 'master' into import-module
shutdown the active crawler on SIGINT. fixes #450
Merge pull request #445 from darkrho/import-module

Use `importlib.import_module` instead of `__import__`
Django 1.6 form validation errors now include ValidationError exception instances instead of just strings
replaced __import__ by importlib.import_module.

Since python 2.7, importlib.import_module is the recommended way to
import modules programmatically.

From __import__'s doc:

    Import a module. Because this function is meant for use by the
Python
    interpreter and not for general use it is better to use
    importlib.import_module() to programmatically import a module.
Replaced remaning __import__(module) calls.

This commit replaces the statements __import__(module) as the previous
replaced the statements __import__(module, {}, {}, ['']).

At first I thought leaving the single-argument calls, but perhaps it's
better to be strict rather than having exceptions to the rule in this
case.
Django 1.6 form validation errors now include ValidationError exception instances instead of just strings
map travis-ci matrix to tox environments
install updated pypy from ppa
TOXENV is tox supported env
building Pillow with pypy requries dev headers
Merge branch 'travix-toxed'

Conflicts:
	requirements.txt
Merge 0.20 release notes
bumped version to 0.21.0
improve 0.20 release notes

Conflicts:
	docs/news.rst
process_parallel was leaking the failures on its internal deferreds.  closes #458

DeferredList implemented cancellation in Twisted 13.2.0 by holding a
reference to the affected deferreds objects, if a deferred errored the
result was propagated to the DeferredList but still referenced by the
original deferred and nobody was consuming it.

The tests started to fail because the reference from DeferredList
prevented the underlining deferred from been collected before the test
finish invalidating the effect of self.flushedLoggedErrors() call.
duplicate 'use' in line 87