First you need to source a decent Stepper Motor. You need one with a very low resistance in the coils, this should give you the Amps required.
Mine had 2.88volts and 2.4A on the label, the resistance was 1.2ohms across the coils. On testing this Stepper motor put out 500mA when shorted with the multimeter at 400rpm!!.
At 15km/h the average bike wheel will be doing around 160 to 200 rpm. That means the motor could be mounted near the hub of the wheel reducing the effort to drive it, unlike the old dynamo running on the tyre at a 1000rpm.
Secondly, you need to purchase a couple supercapacitors which could be found herehttp://www.altronics.com.au/index.asp?area=item&id=R4945
Thirdly, you'll need
-x8 1N4004 diodes,
these will rectify the AC generated by the stepper motor to a DC supply.
-x1 LM317T Voltage Regulator, http://www.reuk.co.uk/Using-The-LM317T-To-Regulate-Voltage.htm
-x1 0.1uf ceramic capacitor (suggested by LM317 datasheet)
-x2 Resistors for the LM317T, a 240 ohm and a 820ohm 1/4watt will give you the 5.5 volts needed for the capacitors, this should not be exceeded!!
-x1 LED 1 Watt http://www.jaycar.co.nz/productView.asp?ID=ZD0424&CATID=33&form=CAT&SUBCATID=917
-x1 Resistor for the LED, 11ohms 1/2watt, this creates a current draw of about 160mA.
Other items to find are
a torch/headlight case to fit the LED into.
a small case to protect the capacitors from damage.
A 100mm dia. plastic drain pipe approximately 25mm wide, epoxied to the spokes of the wheel.
A pulley or rubber wheel for the stepper (approx 50mm dia) and
tools, soldering iron, and what ever is required to fit it to your bike.