Adding model Micropost

Signed-off-by: Rodrigo Dias Ferreira <rodrigodias@gmx.de>
Migrating database

Signed-off-by: Rodrigo Dias Ferreira <rodrigodias@gmx.de>
Adding association between microposts and users

Signed-off-by: Rodrigo Dias Ferreira <rodrigodias@gmx.de>
Adding constraints to models users and microposts associations

Signed-off-by: Rodrigo Dias Ferreira <rodrigodias@gmx.de>
Adding validations to microposts

Signed-off-by: Rodrigo Dias Ferreira <rodrigodias@gmx.de>