Add support for custom error pages in 'nginx'

You can define a set of error pages in nginx server configuration (they
will be automatically protected from being accessed directly from the
outside).
Add custom redirect section in 'nginx' config

This patch adds 'item.redirect_from' key in nginx server configuration.
You can set it either with True/False or a list of domains and names you
want to redirect.

'nginx' role will create custom server section for redirected names and
redirect them to first domain listed in 'item.name' key. If
'item.redirect_from' is set to True, additional domains will be taken
from 'item.name' list; if it's set to a list of names, these will be
used instead.
Update CHANGELOG.md
Merge pull request #122 from drybjed/role/nginx

New variables in 'nginx' role, updated CHANGELOG.md