Add JSONIFY_END_WITH_NEWLINE config variable

When `JSONIFY_END_WITH_NEWLINE` is set to `True`, `jsonify` responses will be terminated with a newline character.

Defaults to `False` for backward compatibility.

This came up in the context of https://github.com/kennethreitz/httpbin/issues/168
Good point. Here ya go:

```
❯ git commit -av --amend
[JSONIFY_END_WITH_NEWLINE d9402fc] Make `jsonify` terminate responses with a newline
 Date: Sat Nov 29 08:58:50 2014 -0800
 3 files changed, 17 insertions(+), 8 deletions(-)

❯ git push -f msabramo JSONIFY_END_WITH_NEWLINE
Counting objects: 4, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (4/4), done.
Writing objects: 100% (4/4), 593 bytes | 0 bytes/s, done.
Total 4 (delta 3), reused 0 (delta 0)
To git@github.com:msabramo/flask.git
 + 410d853...d9402fc JSONIFY_END_WITH_NEWLINE -> JSONIFY_END_WITH_NEWLINE (forced update)
```