Add mysql and postgresql dependencies and shortcuts

README.md

 
     pip install --upgrade pip setuptools
 
-Install the project in editable mode with its testing requirements:
+Install the project in editable mode with its testing requirements :
 
     pip install -e ".[testing]"
 
+If you want to use postgresql, mysql or other databases
+than the default one: sqlite, you need to install python driver for those databases
+that are supported by sqlalchemy.
+
+For postgreSQL and mySQL, those are shortcuts to install Tracim with test and
+specific driver.
+
+For PostgreSQL:
+
+    pip install -e ".[testing,postgresql]"
+
+For mySQL:
+
+    pip install -e ".[testing,mysql]"
+
 ### Configure Tracim_backend ###
 
 Create [configuration file](doc/setting.md) for a development environment:

setup.py

     'mypy',
 ]
 
+mysql_require = [
+    'PyMySQL'
+]
+
+postgresql_require = [
+    'psycopg2',
+]
 # Python version adaptations
 if sys.version_info < (3, 5):
     requires.append('typing')
     zip_safe=False,
     extras_require={
         'testing': tests_require,
+        'mysql': mysql_require,
+        'postgresql': postgresql_require,
     },
     install_requires=requires,
     entry_points={