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
Please change this to: "Templates are no longer automatically reloaded outside of debug mode. This can be configured with the new ``TEMPLATES_AUTO_RELOAD`` config key." Or something similar. The idea behind that rephrasing is that the important change (Flask's behavior changes) goes first, and then the user is told how to revert to the original behavior.