Make Position value class final

Position class is now final, preventing inheritance from it.
Add Identifier value class

An Identifier is just a positive integer (represented via a Value Class) that
will be used in all Entity classes that need to have an ID. Having this class
frees all the Entity classes that need an ID of the responsibility of checking
if a given Integer (maybe passed in in its constructor) is a valid ID
(positive).

The one and only thing that Identifier does is hold a positive integer, and of
course validate that its constructor parameter is one of those.
Make Document use Identifier as ID and Sentence as title

Document is modified to be a final class (disallowing inheritance from it), to
use an Identifier value class as its ID instead of a plain Integer, and to use
a Sentece class as its title instead of a plain String. Using an Identifier for
ID frees Document from the responsibility of checking if a given value is a
positive Integer, and using a Sentence as Title frees it from checking if it is
empty and the future need of trimming white spaces in it (to cleanly store it
in the database).
Add SLF4J-Nop and ScalaMock dependencies

Slick uses SLF4J for its own debug logging, so an implementation is needed.
SLF4J-Nop provides an implementation that just does nothing, so it actually
disables logging from Slick. This is good actually, since it is not needed at
the moment. It will be replaced in the future by another logging framework if
its outputs were needed.

ScalaMock is a framework for creating Mock objects (for testing purposes) for
Scala. It has ScalaTest support, so that it can be both easily integrated.
Current ScalaMock does not have a compiler plugin like previous ones (for Scala
2.9 and older), so it is a bit more limited, but it will probably be enough.
Add Annotation entity class and tests

An Annotation represents an annotated named entity in a document, meaning a
sentence that has been recognized by a NER tool inside a document text. It
holds an identifier, the original sentence, that same sentence normalized (to
an standard representation, eg: "person", "boy", "human"... all could be
normalized to "homo sapiens"), a Category (as defined by the entity
Enumeration) that holds the type of the annotated sentence, a reference to the
document that was annotated with it and absolute start and ending positions of
the original sentence in the document text.