Add a :download command.
Add a raise_cmdexc_if_invalid to urlutils.
Check URL validity in :download.
Add some assertions for 'successful'.
downloads: default autoclose=True. Fixes #265.
Never get downloads from the cache.

This is a workaround for corrupted files because Qt somehow reads them from the
cache wrongly. See https://bugreports.qt-project.org/browse/QTBUG-42757

This might fix a part of #78 - but we'll leave this open for now, since it
might also be another issue.
Ignore 'content-type missing' Qt warning.
Simplify utils.log.

The overcomplicated parsing with re didn't really give us much benefits, and
lead to exceptions and segfaults[1] (?) already, so it's best to just get rid
of it entirely.

[1] http://paste.the-compiler.org/view/26768694
Typo: s/similiar/similar/g
Update quickstart
Add a dest-parameter to :download.
Update docs.
Add a new option tabs->tabs-are-windows.
Handle OSError when using subprocess.
Split some functions into smaller ones.
Fix exception handling in asciidoc2html.
Update commands.asciidoc.
Handle HTTP redirections in downloads.
Add basic adblock functionality.
Use statusbar messages for adblock