Set TEMPLATE_AUTO_RELOAD default value to None

In accordance with the proposal #1214 the default value of `TEMPLATES_AUTO_RELOAD` is set to `None` and by default templates will be reloaded only if the application is running in debug mode. A complete table of the various options:

`TEMPLATES_AUTO_RELOAD`|`DEBUG`|Templates auto reloaded?
-----------------------|-------|------------------
`None`  |`False`    | no
`None`  |`True`     | yes
`False` |`False`    | no
`False` |`True`     | no
`True`  |`False`    | yes
`True`  |`True`     | yes
I think the first sentence can be shortened to:

>Whether to check for modifications of the original file and reload automatically.