Added script for generating BPF asm and Lua code generated by libpcap and pflua.

- Generate BPF.

        $ pflang-compile --bpf "tcp port 80"

- Generate libpcap Lua code.

        $ pflang-compile --lua "tcp port 80"

- Generate pflua Lua code.

        $ pflang-compile --pflua "tcp port 80"

The flags are not exclusive, they can be combined to get several outputs
at the same time.

Some code reorganization within 'tools/' folder.
Merge pull request #38 from Igalia/compilation-script-to-lua

Added script for generating BPF asm and Lua code (libpcap and pflua)
Implement symbolic constants.

- Symbolic constants in arithmetic expressions. Add support for
constansts: 'protocol header field offsets', 'icmp type field values'
and 'tcp flag field values'.

- Symbolic constants in logical operations. Add support for constants
names for port numbers. Valid in 'port <port>' and 'portrange <port-port>'
contexts.

This patch solves issue #24.