Regarding getting the 5VDC from a wall switch. As has been already reported it is:
1 Dangerous if you do not know what you are doing, and
2 Typically a switch may not have a neutral wire available, and
3 Both the wires attached to the switch terminals are on only one leg of the AC.
It can be done, however, in certain circumstances with something similar to this diagram. This circuit will work by running a small amount of power through the light itself (caveat: this circuit was designed when all lights were incandescent, probably will not work with flourescents). The wall wart power module can be wired across the switch and as long as the switch is not turned on the power for the wall wart will be obtained through the light acting as a very low resistance inline resistor. The power consumption must be tailored to the needs of the wall wart without affecting the light.
This circuit was used in a film developing lab to prevent someone opening the door to a darkroom unless the inside light was on. Just do not install the two extra switches and instead of a transformer and bell use a wall wart DC adapter.
Again, if you are not sure, DO NOT DO IT.