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.
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.
Renamed to SimplifiedRadio according to the new naming scheme.

Renamed Radio to SimplifiedRadio.
Renamed ChannelControl to SimplifiedRadioChannel.
Renamed IChannelControl to ISimplifiedRadioChannel.
Renamed ChannelAccess to SimplifiedRadioChannelAccess.
Renamed AirFrame to SimplifiedRadioFrame.
Renamed propagation directory to attenuation.
Followed renames in comments and variable names.

Renamed airFrame to radioFrame.
Renamed channelControl to radioChannel.
Followed renames in examples and tests.
Adapted IdealRadio to the new radio interface.