add set_pull method to pin class

[johndela1]

This change adds a set_pull function to control pull resistors.

see: #288

[dpgeorge]

DId you test that this works? I think the DAL will override the setting once you call read_digital.

[johndela1]

This is the behavior:

>>> pin1.set_pull(pin1.PULL_UP)  
>>> pin1.read_digital()        
1
>>> pin1.read_digital()
1
>>> pin1.set_pull(pin1.PULL_DOWN)
>>> pin1.read_digital()          
0
>>> pin1.read_digital()
0

now if you call read_analog then call read_digital it will set it to PULL_DOWN

with this code:

 // Move into a Digital input state if necessary.                                           
    if (!(status & IO_STATUS_DIGITAL_IN)){
        disconnect();
        pin = new DigitalIn(name,PullDown);
        status |= IO_STATUS_DIGITAL_IN;
    }

[markshannon]

We now have a set_pull method on pins.
@johndela1 thanks for your efforts