Implemented creation/deletion of databases (PUT /db, DELETE /db)

This doesn't actually change anything in the bucket, just hooks a
bucket up to the gateway or unhooks it.
Note: This commit isn't thread-safe. Some more work is needed there.
Part of #102
Refactored serverContext struct

* Moved into its own file server_context.go
* Made public
* Removed unnecessary struct 'context' (this was messier than you'd think)
Made ServerContext thread-safe by locking access to .databases map

…now that it's mutable.
Some cleanup of ServerContext
Log username (or admin status) along with request URL

Also make sure that 404/405 errors and OPTIONS requests get logged.