Updates to basic_tunnel and basic (#78)

* Adding initial implementation of basic_encap example

* Updated basic_encap example to count the number of valid packets

* Updated basic_encap example to put encapsulation layer after Ethernet
header.

* Added solution file for basic_encap example

* Changed the name of the basic_encap example to basic_tunnel and called
the new header myTunnel. Also changed the myTunnel field names slightly.

* Updated the README file for the basic_tunnel exercise. Also added topo.pdf
image to serve as a reference during implementation.

* Updated basic/README.md to point to basic_tunnel as the next exercise.

* Updated the README for basic to point to basic_tunnel.

Updated the starter code for basic_tunnel to look like basic
solution with todo comments.

Updated send.py and receive.py to be able to send both plain IP
packets and tunneled IP packets.

Updated basic_tunnel.p4 to have same control flow as p4runtime
exercise.

P4D2_2017_Fall/exercises/basic/README.md

@@ -140,7 +140,7 @@ Other questions to consider:
 
 There are several problems that might manifest as you develop your program:
 
-1. `basic.p4` might fails to compile. In this case, `run.sh` will
+1. `basic.p4` might fail to compile. In this case, `run.sh` will
 report the error emitted from the compiler and halt.
 
 2. `basic.p4` might compile but fail to support the control plane
@@ -167,6 +167,7 @@ make stop
 
 ## Next Steps
 
-Congratulations, your implementation works!  Move on to the next
-exercise: implementing the [basic tunneling](./basic_tunnel)!
+Congratulations, your implementation works! In the next exercise we
+will build on top of this and add support for a basic tunneling
+protocol: [basic_tunnel](../basic_tunnel)!