Ideas 31 resourceproxy max file size configurable

Addressing roadmap [issue #31](https://github.com/ckan/ideas-and-roadmap/issues/31), this introduces a config-based resource_proxy.max_file_size.

* The config template has now a new, commented-out, section "Resource Proxy" with setting ckan.resource_proxy.max_file_size showing the default value
* The Resource Proxy controller seeks this setting, or defaults to the previously hard-coded 1MB.

Next steps:
* Write tests
* Determine sane limits for max_file_size
* Handle errors caused by extreme max_file_size settings with informative log messages