Bundle libnet

This was originally discussed in #91.. Sorry I took so long :-/

I have the usage of bundled libraries (right now, just libnet) enabled by default. libnet will be statically linked into the binary, so we're not dropping new shared libs onto the system. 

Maintainers who want to disable bundled libraries completely should set -DBUNDLED_LIBS=Off (That's for you, @LocutusOfBorg)

I can do the same thing with any of our dependencies (libpcap, openssl, curl, etc).

From INSTALL:

```
#### Bundled libraries
Ettercap now bundles the following libraries with the source distribution:
  libnet 1.1.6

We will build bundled libraries that Ettercap depends upon, so you 
don't have to! 

By default, the build system will search for system-provided libraries. If it 
doesn't find the particular library it wants, it will build the library, itself.

# To disable the use of bundled libraries:
  $ cmake -DBUNDLED_LIBS=Off ../

# To disable the searching for system-provided libraries, and use 
# bundled libraries exclusively:
  $ cmake -DSYSTEM_LIBS=Off ../
```