Model entities refactor and cleanup

All model entities have been moved from the "*.model" package to "*.entity",
to have a simpler package structure. Also, all the subdirectories
"es/uvigo/esei/tfg/smartdrugsearch" have been deleted, since Scala does not
require to have a subdirectory per subpackage, so "package
es.uvigo.esei.tfg.smartdrugsearch.entity" can still be used, but only a
"entity" directory is used for all the package hierarchy.

Category has been changed from a Scala Enumeration to a Sealed Trait and Case
Objects that extend it. It's a bit more of code, but easier to maintain, and
cleaner. And NamedEntity has been renamed to Keyword, since it is a better name
for what that class represents.

A empty "util" package object has also been created, to be filled by util
methods and objects when needed.
Database refactor and cleaup

Database access classes and objects have been refactored, improving its design
by not having a big DAL class that holds everything. Separate DAOs are now used
for each database table, a DatabaseProfile class holds the database connection
via a JdbcProfile, and mixing in Mappers, Profile and a new Tables trait that
defines the different database tables.

Also included new tests for DocumentsDAO, and commented out all code inside the
annotator package, since it needs to also be refactored to use this new
database package design.
Add KeywordsDAO and tests

Implemented KeywordsDAO for KeywordsTable and its tests.
Add AnnotationsDAO and tests

Implemented AnnotationsDAO for AnnotationsTable and its tests.
Annotator package refactor and cleanup

NERAdapter and LinnaeusNERAdapter have been updated to use the new database
package structure: they now have an empty constructor, create a DatabaseProfile
via its factory companion object, and use the existing DAOs instead of
operating directly on a TableQuery object. Also, all previous naming refering
to "NamedEntity" has been replaced by the new name "Keyword".

Also updated OscarNERAdapter test to the new NERAdapter structure, but not
implemented yet the Oscar adapter.