Added robots.txt to disallow searches
Added ETag HTTP caching call in MyResponse

Change due to issue #12. Config file now contains 'HTTP_CACHE'
boolean constant. When set to true all HTTP responses that are
applicable for caching will contain the ETag header.
The ETag header is generated from the same key that is used to
cache results in the Redis cache server.
Refactored Cache classes inheritance model

Added abstract CacheBase class that implements CacheInterface.
This class contains common cache functionalities like the genKey() method.
Bumped version in composer.json to 0.9.1