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.
pylint cleanup: unused imports and old-style exceptions
restored import *, added comment
Merge pull request #430 from alexanderlukanin13/cleanup

pylint cleanup: unused imports and old-style exceptions
PEP8 improvements

* Space lines around classes/functions.
* Space in inline comments.
* Remove redundant backslash between brackets.
Python 3 compatible syntax: print, except, raise, octal numbers; removed Python 2.2 boolean compatibility code in xlib/pydispatch/dispatcher.py
Removed reference to URL in crawl command as it's no longer supported.
DOCS removed .tld suffix for spider names for the sake of consistency.
Added "from __future__ import print_function" in all relevant places except xlib
Merge pull request #432 from darkrho/crawl-url

Removed URL reference in crawl command and .tld suffix in docs for spider names
simplified description of crawl command
Converted map() to list comprehension or generator for Python 3 compatibility
Using next() for Python 3 compatibility
Merge pull request #435 from alexanderlukanin13/syntax-map

Converted map() to list comprehension/generator for Python3 compatibility
Merge pull request #436 from alexanderlukanin13/syntax-next

Using next() for Python 3 compatibility; ordereddict.py converted from DOS to Unix
print_function in xlib
Merge pull request #431 from alexanderlukanin13/syntax

Python 3 compatible syntax: print, except, raise, octal numbers; removed...
debian package requires python-cssselect
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.