Introduce basic LINC-Switch integration

LINC-Switch is integrated with mininet by introducing a new class that extends
Switch. For the time being LINC-Switch support is not fully tested a need
to be extended so that all mininet features can work with LINC. To start
mininet with LINC simply run the mininet executable as follows:
`sudo mn --switch=linc`.
(Motivated by FlowForwarding/LINC-Switch#129).
Add LINC-Switch installation option to the installation script for Ubuntu

Now LINC-Switch (https://github.com/FlowForwarding/LINC-Switch) can be installed
on Ubuntu along with the Mininet and its dependencies via the installation script.
The script also installs LINC's dependencies: erlang, uml-utilities
(that provides tunctl), bridge-utils (that provides brctl) and git-core.
(Motivated by FlowForwarding/LINC-Switch#129).
Include information on LINC-Switch support in the README