feat(http): add basic http service

[jeffbcross]

TODO:

• Add documentation comments
• Open issues to capture remaining features for current milestone

This implementation only works in JavaScript, while the Observable transpilation
story gets worked out. Right now, the service just makes a simple request,
and returns an Observable of Response.

Fixes #2028

[PatrickJS]

the pattern seems to be httpInjectables
https://github.com/angular/angular/blob/master/modules/angular2/router.ts#L33
https://github.com/angular/angular/blob/master/modules/angular2/router.ts#L33

could you also export Http, HttpFactory, XHRBackend, and BrowserXHR for users who want to overwrite/replace/remake etc

[jeffbcross]

Thanks for all the comments, @gdi2290!

[jeffbcross]

remove args for symmetry

[jeffbcross]

@vsavkin did you have any other feedback other than what we captured on our call? (If not, can you add lgtm label?)

[vsavkin]

@jeffbcross no, just make Travis happy!

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[jeffbcross]

Merged at 2156810

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