Add HTTP input plugin

[fujifish]

Add HTTP input plugin for the server to be able to receive incoming logstash messages over HTTP/HTTPS.
This is needed in situations where the agent resides in a location that is only able to access HTTP/HTTPS, e.g. behind a proxy.

[bpaquet]

Thx you for this big contribution !

I can not merge it as is. I will implement, using your code

• UTF8 as default encoding (started in 224acfc)
• Do not load optional dependencies when not needed
• Add support for old logstash format support (issue Add filter to import old json logstash format #40)

After that, we can merge your HTTP input plugin !

Bertrand

[bpaquet]

@fujifish I have implemented the three points.

You do not need for commits 2b2aaa1, 906400f, 3633018.

You can merge it by yourself, or I can do it for you. What do you prefer ?

[fujifish]

Go ahead and do the merge.
I've also added support for tunneling the http_post output through a proxy and it would be great if you can merge it as well (a4fdce7, 7141058, b2cf230, e83b576)

[fujifish]

I've merged your changes back, so I'll close this pull request and open a new one