Refactor lanattacks ruby code, add command dispatcher

The lanattacks module didn't seem to have a command dispatcher, and
hence loading the module would always result in a failure. This
commit fixes this problem.

The commit contains a bit of a refactor of the lanattacks code to be
a little more modular. It also has a shiny new dispatcher which breaks
the DHCP and TFTP functionality up into separate areas.

Loading the module in x64 and x86 succeeds. Here's a sample:

```
meterpreter > use lanattacks 
Loading extension lanattacks...success.
meterpreter > help

... snip ...

Lanattacks: DHCP Commands
=========================

    Command            Description
    -------            -----------
    dhcp_load_options  Load DHCP optionis from a datastore
    dhcp_log           Log DHCP server activity
    dhcp_reset         Reset the DHCP server
    dhcp_set_option    Set a DHCP server option
    dhcp_start         Start the DHCP server
    dhcp_stop          Stop the DHCP server


Lanattacks: TFTP Commands
=========================

    Command        Description
    -------        -----------
    tftp_add_file  Add a file to the TFTP server
    tftp_reset     Reset the TFTP server
    tftp_start     Start the TFTP server
    tftp_stop      Stop the TFTP server

... snip ...
```