Replace repoze.what-pylons dependency with custom function (fixes #250)

Introduce a direct permission check `perm`, which can be used together
with `can`, `check` and `require`, e.g.

    require.perm('global.admin')

Additionally, introduce a `guard` decorator, which allows to execute a
function in advance of the decorated function, e.g.

    @guard(require.perm, 'global.admin')
Use adhocracy_Pylons in paster plugin configuration
Make `guard` a decorator which behaves as `require` does
Garden

* Remove unneeded imports
* Make require call as early as possible
Merge pull request #252 from nidico/feature-250-remove-repoze-what-pylons-dependency

Replace repoze.what-pylons dependency with custom function (fixes #250)