Creating User model

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

Signed-off-by: Rodrigo Dias Ferreira <rodrigodias@gmx.de>
Validate name length, email format and uniqueness

Signed-off-by: Rodrigo Dias Ferreira <rodrigodias@gmx.de>
Adding index to email column,  guaranteeing uniqueness, and lowercase callback

Signed-off-by: Rodrigo Dias Ferreira <rodrigodias@gmx.de>
Add password digest + Guardfile to include test.sqlite3

Signed-off-by: Rodrigo Dias Ferreira <rodrigodias@gmx.de>
Adding password, confirmation, has_secure_password

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