Use curl constant instead of string names to retrieve execution result

Signed-off-by: Stéphane HULARD <s.hulard@chstudio.fr>
Coding style update

Signed-off-by: Stéphane HULARD <s.hulard@chstudio.fr>
PHPDoc update

Signed-off-by: Stéphane HULARD <s.hulard@chstudio.fr>
Update client coverage

Signed-off-by: Stéphane HULARD <s.hulard@chstudio.fr>
Add AbstractMessage test

Signed-off-by: Stéphane HULARD <s.hulard@chstudio.fr>