Add Virtuslab Unicorn dependency

Unicorn is a small library that enables writing type safe primary and foreign
keys in Slick applications. It is written to be used with Slick 1.x, support
for Slick 2.x is in development (Slick 2.0.0 has been released just a month
ago), so Slick dependency is downgraded also to 1.0.1 to accommodate.

Also added Play-Slick dependency, a library that makes Slick a first class
citizen inside the Play Framework, and established JDBC as a dependency to Play
Framework.
Add Play Framework config file

Application.conf inside "conf" directory establishes the configuration for Play
Framework. The only important stuff there is the configuration of the database:
default one has been set to a MySQL DB in localhost, and test is set to a H2
Database (in memory); and the final line that tells Slick where to look Scala
files that define tables to generate Play Evolution's SQL Files.
New model package structure and refactoring

Entity classes have been moved to *.model in favor of using a *.mode.entity
package, since it will be simpler this way. Category, Position and Sentence
also have a new implicit definition in its Predef sub-objects that enable
to/from database type conversions via Slick's TypeMapper.
Remove Identifier and modify Document and Annotation models

Identifier class is no longer needed, since Unicorn library provides a type
safe BaseId and IdCompanion traits that can be used to define Identifiers for
each model that needs them. So it has been replaced by DocumentId and
AnnotationId. Annotation and Document classes are modified to be in sync with
the deletion of the Identifier class.

Defined Documents and Annotations SQL Tables via Slick Table definition. These
are subject to change since I think they are not properly defined: maybe split
Annotations table in two, one that relates Documents with Named Entities and
holds the original text and positions and another that defines the Named
Entities and holds just an ID and the original text.