Add default picture to device (Fixes #39)

Uses a '?' as the default picture for devices in case child classes don't
implement their own.  Decided to leave the thrown error if picture not
found as this is an actual error that needs to be fixed, and there is not
appropriate error indicator to connect the VI. The user has the option to
press continue and ignore the problem, so it won't prevent the use of
SetList even if the PNG file somehow gets deleted.

Fixes #39.
Slave-slave conflict gives chan.# in error message

To help user find problems, the error message now include channel number
information instead of just stating there was a collision.  Fixes #40.
Update default Trigger Channel to 65535. Fixes #38

Changes the default trigger channel to 65535 (Max unsigned 16-bit value)
for new devices to help make errorneously unprogrammed trigger channels
obvious.
Master requires Check_Reserved_Lines override

Because this is part of our device collision error checking routine, we
require all master devices to implement the Check_Reserved_Lines VI.

Fixes #45
Merge pull request #46 from CHerold/Fixes-Held-for-Freeze

Fixes held for freeze; other work finished so freeze is over.