refactor create/update/destroy of listeners

Don't close and reopen listeners

This prevents a deadlock:

1. rehash() is executing on the main goroutine
2. it's trying to stop a listener goroutine
3. the listener goroutine needs to hand off a new connection to newConns
4. but the main goroutine is blocked by rehash() so it can't receive it

See #134; there was a `fatal error: concurrent map read and map write`
due to unsynchronized accesses to `Server.listeners`. Now, `listeners`
is only accessed by `NewServer` and `rehash`, so it doesn't need
synchronization.

implement #132 (configurable default channel modes)

fix hostmask issue

PTR records ending in . (e.g., `google-public-dns-b.google.com.`)
were considered invalid

server: close connection on parse-ip failure

strings: disallow ':' in nicks

I don't think casefolding things like `:6697` ever made sense.
Since these are configured by the ircd operator, it makes sense to
assume they'll already be in a canonical form regardless.

This matches the behavior of inspircd at the very least.

Previously, the comment above that section claimed ':' should be
disallowed, but the code didn't do so.

I also simplified the code a little bit and added tests.

Close the client's connection if we're unable to parse their IP.

This also simplifies the check to reduce indentation by a level.

Finally, this replaces the two-var construction of the pseudo-const
messages with an inline dereference via a slice to allow constructing
them less noisily.

simple travisci definition

This is a gotcha called out in the `time.After` docs. `time.After` will
leak the underlying channel if nothing ever receives on it.

None of them are referring to actual files

The variables were often referenced with a trailing slash, and having
`//` works, but is sloppy.