get configure from a url

This pull request allow fluentd get full configure from a url such as an API of SCM

The configure content could be dynamic generated by a managment system.
It would be very useful if we have a large number of machines and different configure on each machine. 

Two new params has been added:
-u remote_config
-t timeout

ps:
I got a error when run rake install 
ERROR:  Error installing ./pkg/fluentd-0.10.32.gem:
	fluentd requires fluentd (>= 0)

I don't know how to fix this, so I switch jeweler to bundler https://github.com/fluent/fluentd/commit/ede5a1f42603da86c3b8dab980fe924c43709fb6