Stubbed out basic directory structure.

Added [rebar](https://github.com/rebar/rebar) version 2.3.1 to the
repository.
Created application resource file.

Nothing exciting going on yet.
Defined `chkrec_keeper' module.

Implements the `gen_server' behaviour. Allows another process to store
and retrieve a value from this process.

We use a `maybe/1' type to represent an optional value. If another
process were to use this code for backing up its state, then during
its initialization the call to `get/1' would return `none' indicating
that no backup has been made yet. Otherwise `get/1` would return
`{value, Value}', where `Value' is the last store value using `put/2'.
Created supervisor for `chkrec_keeper`s.

The `chkrec_keeper_sup' uses a `simple_one_for_one' restart policy,
since its children are all `chkrec_keeper's which vary only in their
`Source' argument, i.e. the process they are backing up.
Made registry of `chkrec_keeper`s.

Provides a convenience function to spawn new `chkrec_keeper`s under
the supervision of the `chkrec_keeper_sup` process, while maintaining
a mapping of the identifiers for useful workers to the pids of the
backup keepers. A useful worker can employ this registry to lookup the
pid for the backup keeper and store/retrieve its state.

Note that if for any reason the `chkrec_keeper` were to go down, the
information stored in registry would be stale. This will be fixed in
a later commit.