Serv-Arm improved for easier assembly

This is a remix of the very good Serv-Arm by Heartman. Thanks for this great design!

We use the Arm in our Makerspace for education and it works pretty well.

Changes:

- Added holes to the base to screw the arm to the ground if neccessary.
- Made all servo holes smaller so one can mount the servos with small screws. No need for M3 or M4 screws. Just use the screws that come with the servos.
- Made the gripper a little bit thinner so it bends more easily.