Simple authentication with sessions/configuration file.

This is not the end goal, this is just something to get the web
security started. I've added two new additions to the json config
file:

```json
...
"Username": "admin",
"Password": "admin",
...
```

Obviously you should _not_ be using `admin:admin` as your user/pass
combination, but that's not up to me what other people use. The end
goal is to use _some sort_ of database (ref #45) for usernames and
passwords, and possibly allow multiple users and let each user have
their own servers/channels with their own information.

I also added some comments in other places that I did not have prior
to this commit, and removed some empty lines. The web interface now
has a "/login" page and everything gets redirected to it if a user
is not logged in and trying to access something else.

The next thing that it should do is forward 404 requests if not logged
in so it doesn't show what pages exist and what pages do not exist.

Ref #32
Renaming the session name to config.Username instead of "admin"

I was using "admin" for testing, but it really shouldn't just
be admin. I'm not 100% sure what the session name should even be.
I was thinking it should probably be a long user identification
string of some sorts, but I've yet to figure that out. I need to
do a little research on exactly _how_ sessions should be named
properly before I fix this.
Removed extra space
Adding comments for IsLoggedIn func
GetSessionName is a func that gets the session name.

Right now it's just returning "user" as the session name. I'm not
sure if it should be doing something else to get a session name,
so I'm just returning that for now. If there's a better way to do
it then we can in the future by easily updating the func.
Adding a logout and a few other things:

* Adding a label for username/password because some browser support
* Added a /logout route
* Remove cookie/session on GET (or any request, really) to /logout