Reduce size of server.js, moving code to their related files

This PR accomplishes two things:

*  Moves all routing related code to the newly created `core/server/routes` folder.
*  Moves all middleware related code to the newly created `core/server/middleware` folder.

This helps reduce the size (LOC) and purpose of `server.js`, one step closer to a better simplified initialization of the ghost app (refer to task in https://github.com/TryGhost/Ghost/issues/360#issuecomment-27690186 ).

A little more invasive than my last PR but hopefully helpful to ghost at large.  I've left this PR as two commits for review purposes in case both changes aren't desired.

Let me know what edits you desire!