Make top-level repo an application

The top level repo should be what selects and pulls together the
indivual applications that make Apache CouchDB. The couch application
itself should become more of the engine that powers the core but does
not decide or determine what is built around it.

This is a start to moving away from the .ini driven boot process.
Dynamic application support is an interesting feature that might
belong but it's something that makes it hard to clean up the rest
of the code. Over time, more of that might be added back but first
a clean OTP-style boot and clean rebar based building and embedding
would be better trade-offs.

There will be follow up commits that move more code around as this
idea settles. Tests are currently a bit broken. These will be fixed
separately as well given how much separate work they will require as
is.