Add implicit Extensions to database Components

Implicit classes serving as extensions to the default Slick database queries
are added to be able to compose more complex queries involving Documents and
Named Entities (Annotations not yet implemented) with the same ease of use as
the default Slick ones.
Add Twitter util-collection library

Twitter's util-collection library for Scala is added as a libraryDependency to
the project. Currently, it will only be used for its LRUMap implementation,
which will be useful to implement a very simple and small cache.
Implement NERAdapter base Trait

Added a small implementation of a NERAdapter Trait that all NER Tool adapters
must conform to (and extend). It implements an Akka Actor that can receive
documents and annotate them storing its results in the database.

Implementations of this trait mus provide a DAL object as attribute (probably
received in its constructor), and its database Session should be established by
sending a Session message to the actor. They also should implement the
annotate(Document):Unit method providing the annotator logic required to parse
the document and store its NamedEntities and Annotations in the database as
expected.
Add Akka Testkit dependencies

Upgraded Akka to 2.2.3 from whatever version Play had as an internal
dependency, and added the Akka TestKit as a test dependency.
Fix uvigo link in README