initial category support, part of #121

Add support for categories to Acrylamid. Categories are either explicitly set
in the metadata section (`category: foo/bar`) or derived from the path of the
post, e.g. `content/projects/python/foo-bar.txt` will set the category to
`projects/python`.

    content/
    ├── projects
    │   ├── bla.txt
    │   └── python
    │       └── fuu.txt
    └── test
        └── sample-entry.txt

The directory structure above and the following conf.py configuration

    '/category/:name/': {'view': 'category', 'filters': 'summarize',
                         'pagination': '/category/:name/:num/'},

renders the following on `$ acrylamid compile`:

     create  [0.34s] output/2013/i-am-only-in-python/index.html
     create  [0.02s] output/2013/i-m-a-projects-post/index.html
     create  [0.01s] output/2012/die-verwandlung/index.html
     create  [0.00s] output/category/test/index.html
     create  [0.00s] output/category/projects/index.html
     create  [0.00s] output/category/projects/python/index.html

Both, bla.txt and fuu.txt are shown on the project listing, but only the
fuu.txt post appears on the projects/python listing.

To link to the category listing, use the following Jinja2 code instead of
the tag code:

   {% if 'category' in env.views and entry.category %}
       <p>categorized in
           {% for link in entry.category | categorize %}
               <a href="{{ env.path + link.href }}" rel="category">{{ link.title }}</a>
               {%- if loop.revindex > 2 -%}
               ,
               {%- elif loop.revindex == 2 %}
               and
               {% endif %}
           {% endfor %}
       </p>

This uses the new `categorize` filter which is available when you have the
category view enabled. It takes the category list from `entry.category` and
yields the hierarchical categories, e.g. projects/python yields projects and
projects/python.
Added metadata parameter 'copyres' to provide a mechanism to copy resources (images etc) with the entry
from __future__ import print_function
Updates copyres so files are gathered upfront and available in templates as "respaths" and "resfilenames"
Also creates change events for hooks.
Change sitemap to exclude resources (can add via config SITEMAP_RESOURCE_EXT, excluding images/video)
updated list
fix #160
notice for the master branch
Python 2/3 compatible codebase
mv acrylamid/specs/ to tests/
new workaround the PYTHONHASHSEED issue

only temporarily, better use Adler-32 or something similar.
internal webserver now uses `.send_header` to revalidate, fixes #162

(issue only appeared on Python 3.3)
Merge branch 'feature/unify' Python 2/3 compatibility

EXPECT THINGS TO BREAK!
try zlib.crc32 as internal hash value

This reverts commit 97f945aca92558517168f580f939360202d55e5f.
use OrderedDict for deterministic hashing
fix two minors
fix unittest with removed translitcodec
let Travis CI test with 3.3 (fails due an incompatible test dependency)
drop support for Python 2.6

* even Debian stable now offers 2.7
* ordereddict backport doesn't work well with utils:Struct
more compatibility fixes for Python 3.3
unidecode superseds translitcodec

more ASCII transcriptions and Python2/3 compatible