Add ntb_netaddress_is_ipv6

Adds a function to determine whether an IP address is IPv6 and also
modifies ntb-netaddress.c to use it internally instead of explicitly
comparing against the IPv4 magic.
Make ntb_netaddress_from_string take a default port argument

That way it can later be used to parse addresses for other things such
as the proxy address.
Add an option to use a SOCKSv5 proxy

Adds the -r option which can be used to specify a SOCKSv5 proxy. There
is a separate function in ntb_connection to connect via a proxy. This
uses a new ntb_proxy object to manage the proxy connection state. The
ntb_proxy shares the input and output buffers of the ntb_connection
and the read and write functions of ntb_connection divert to ntb_proxy
until the connection is established.
daemon: Add an option to disable DNS bootstrapping

There was already the -b option to disable bootstrapping altogether.
This adds a -B option which only disables the DNS bootstrapping but
leaves the static default nodes. This is useful for running under Tor
where doing a DNS bootstrap would leak a DNS request outside of Tor.
daemon: Add an option to disabling incoming connections

This adds the -i option to disable adding the listen addresses. This
will be useful when running under Tor where accepting incoming
connections on a public address may give away that the server is
running Bitmessage.
daemon: Add a convenient option to connect to a local Tor server

Adds the -T option which is equivalent to -r 127.0.0.1:9050 -B. This
sets up it to use a Tor server on localhost listening on the local
port and disables DNS bootstrapping.
Update the list of options in the README
Add a note about using Tor to the README