Added first draft of power management for issue #3

[BenjaminPelletier]

Also removed IP constants duplicated in Marshall.h, changed first-time boot to use enableEthernet() and disableEthernet()

A few notes:

• I put SPI pin disabling into the generic disableEthernet() method which is called from other places in the code also; please verify this was the appropriate thing to do
• The timing for when exactly pin 30 is set as an output has been changed to avoid duplicating that code (since pin 30 is set as a LOW OUTPUT regardless of which power mode we're booting into). Please verify this is ok also.