Add file header to idris-core.el
Use Emacs's built-in completions handling for REPL

Now, the same mechanism is used for REPL completion as for in-buffer
completion. This simplifies the code, and it lets alternate completion
interfaces such as ido-at-point also work with Idirs REPL buffers.

Fixes #36.
Merge pull request #114 from david-christiansen/issue/36

Use completion-at-point in REPL