Bumps version to 0.26a1.

[ci skip]
readme: Fixes typo in requirements list.
options.py: Implements choices validation for --restrict-file-names.
document.py: Updates HTMLReader.is_html_file() to match libmagic rules.
Implements --sitemaps. Closes chfoo/wpull#69.
scraper.py: Catches and discards invalid URLs on urljoin.

Closes chfoo/wpull#77.
Adds some IPv6 URL unit tests.
url.py: Preseves whether query strings contain '='.

Closes chfoo/wpull#76
scraper.SitemapScraper: Catches XML parse errors.
Merge branch 'issue/69-sitemaps' into develop

Conflicts:
	wpull/scraper.py
changelog.rst,terse_options.rst: Update for --sitemaps option.
scraper.py: Renames urljoin to urljoin_safe and return None.
url.py: Makes URLInfo.url a property.

Removes redundant URLInfo.normalize. Adds url.normalize() convenience
function.
Merge branch 'issue/76-query-string-delim' into develop

Conflicts:
	doc/changelog.rst
writer.py: Simplifies safe_filename() usage.

url_to_dir_path() renamed to url_to_dir_parts(). Now returns a list.
url_to_filename(), url_to_dir_parts() no longer calls safe_filename().
Implements --max-filename-length.

Closes chfoo/wpull#39.
url.py: RecursiveFilter accepts keyword args.
Implements --span-hosts-allow.

Re: chfoo/wpull#61, chfoo/wpull#66.
Fixes span-hosts-allow unit test URLs to point to correct port.
changelog: Updates latest to 0.26.