Reorganize the code a bit

[bbatsov]

One more "maintenance" PR from me. It doesn't make any "logic" changes - it just restructures the code to follow the best practices for organizing code in Emacs packages. Namely:

• it groups together server-related code
• the minor-mode code (usually it's at the end of each source file)

I've also renamed references to "agent", as we now are using the term "server" instead.

Overall, those changes should make it easier to navigate the code of the package.

[bbatsov]

@zerolfx @jcs090218 Would you mind taking a look at this one? I'd like to get it merged before continuing with smaller incremental improvements here and there.

[bbatsov]

Thanks!