ieee80211Mac: added signals, statistics
linklayer: moved WirelessMacBase to common folder

this class will be shared by:
  - IdealWirelessMac module in src/linklayer/idealwireless
  - Ieee80211Mac module in src/linklayer/ieee80211/mac
idealwireless feature added: code, test, examples
idealwireless: 2nd round of improving the documentation
idealwireless: a little cleanup of the example simulation