Redis LRU cache is added on http requests, only
response with a valid status code are cached during 1 day.

Close: #50

This PR adds a new setting to engines named `tokens`.
It expects a list of tokens which lets searx validate
if the request should be accepted or not.

On dev mode we dont want use tor as proxy by default. With
env. variables these settings are only available on prod.

Close: #44

Ref: #33

Related to discussion in #1124
The io.open import is necessary for python2

SearX currently doesn't start up when run with Python 3 as it tries to parse the
settings.yml file with ASCII codecs.
There are similar problems with engines_languages.json and currencies.json
Python 3 requires that files with Unicode characters be read with a 'b' flag.
This also works with Python 2 and hence can be integrated into the main source
code.

Tested with the latest Python 3.6.4rc1 on Debian unstable.
Signed-off-by: Joseph Nuthalapati <njoseph@thoughtworks.com>

The exact order is
* first from SEARX_SETTINGS_PATH,
* if not found then from searx code base,
* if not found then from /etc/searx/settings.yml
* if not found an exception stops searx loading

* parsing XML-Files which contain target, exclusions and rules
* convert regex if required (is a little hack, probably does not work
for all rules)
* check if target rule apply for http url, and use the rules to rewrite
it
* add pice of code, to check if domain name has not changed during
rewrite (should be rewritten, using publicsuffix instead of little hack)

* add comments
* add licence-header