in_http improvements

the following patches add support for two new configuration directives:

* add_remote_ip : when is set, it stores the ip address through a REMOTE_ADDR field
* cors_allow_origins: set a white list of domains that can do cross requests
We have array type since v0.10.46 so using array is better than 'a,b,c' string style.

http://docs.fluentd.org/articles/config-file#supported-data-types-for-values