Added upload support in HTTP

[n-studio]

I wrote a little patch to enable uploads with the HTTP wrapper.
I'm a beginner so the code may be a bit dirty, feel free to improve it.

It's inspired from ASI HTTP Request.

How to use:

BW::HTTP.post(url, {:payload => {[name] => [value]}, :files => {[name] => [NSData]}}) do |response|; end

Sorry I haven't written tests, I don't know how to do > .<

[n-studio]

Woops, my patch fails when payload is nil and files is not nil

[jimsynz]

@mneorr can you provide any comments/guidance on this PR, since the http code is your baby?

[supermarin]

Actually Matt wrote more than me :)

I'm not comfortable in merging it as-is until we have some specs.
Today I'll try to write them and refactor / merge the code.

@n-studio thanks for the contribution!

[supermarin]

Manually embedded the source, the specs are passing.
I'll merge it now, and then refactor / write new specs.

[n-studio]

Thanks!