Allow specifying server_name parameter values when joining a room

[anoadragon453]

This PR adds a serverName argument to CSAPI.JoinRoom, allowing clients to populate the server_name query parameter.

Additionally useful for homeservers that don't parse server names from room IDs.

Fixes: #39

Aside: I didn't put any newlines between logical blocks of code as the rest of the codebase doesn't do this, even though I sort of wanted to. Is there a general code style for complement defined anywhere?

[kegsay]

Aside: I didn't put any newlines between logical blocks of code as the rest of the codebase doesn't do this, even though I sort of wanted to. Is there a general code style for complement defined anywhere?

gofmt, which allows at most one blank line. I generally use a comment as a separator between logical blocks of code e.g:

// make images
a
b
c
// commit images
d
e
f
// run images
g
h
i

YMMV and I don't care, so long as gofmt is happy.