Added a new physical layer contract.

It is rather a placeholder now than an immediately useful interface.
Added the new radio interface to the physical layer contract.
Added new link layer interface to the contract directory.
Moved to link layer contract.
Added the new network layer contract.
Split network layer contract into upper and lower part.
Added RadioBase to avoid duplicating gates, signals, etc.
Added new power source contract.
Added two power source implementations.
Moved the current radio implementation into a separate directory.

There will be 3 different kind of radios: ideal, simplified and detailed.
Moved the ideal radio into a separate directory.
Moved the old link layer contract into the common directory.

This isn't really a contract in the sense of being the common interface among different link layer implementations.
Moved the IPv4 contract into a separate directory.

This is rather a contract among different IPv4 implementations.
Moved the IPv6 contract into a separate directory.

This is rather a contract among different IPv6 implementations.
Moved the current network layer contract to common.
Moved the echo protocol to common.
Moved the IEEE 802.11 radio into a separate directory under physical layer.

Renamed Ieee80211RadioModel to SimplifiedIeee80211RadioModel.
Fixed NED names in examples and tests.

Followed renames in the models.
Renamed idealwireless to ideal.

Renamed IdealWirelessMac to IdealMac.
Renamed IdealWirelessFrame to IdealMacFrame.
The implementation didn't contain any wireless specific code.
Renamed to IdealRadio according to the new naming scheme.

Renamed IdealAirFrame to IdealRadioFrame.
Renamed IdealChannelModel to IdealRadioChannel.
Renamed IdealChannelModelAccess to IdealRadioChannelAccess.