WIP: Implement HttpService:RequestAsync

[LPGhatguy]

Closes #101.

This PR implements a real version of HttpService:RequestAsync using LuaSocket's socket.http and ltn12 modules.

HttpService can also be constructed using a fake HTTP implementation, which is useful for testing HttpService itself, but may also be useful for consumers of Lemur. Before merging this, perhaps I should also add a configuration option to habitats to disable real HTTP requests or make them cause errors.

Merge checklist:

• Finish tests
• Changelog and FEATURES entries
• Real world test (Rojo integration tests)

[coveralls]

Coverage decreased (-0.6%) to 97.195% when pulling 96d4166 on http into 27b17e3 on master.

[LPGhatguy]

I've talked about this via other channels, but LuaSocket's HTTP won't be enough for this feature. Roblox code can assume that other threads will run while HTTP requests are in-flight, but LuaSocket is synchronous as far as I know.

I prototyped a new, async HTTP interface for Lua over here: https://github.com/LPGhatguy/lua-async-http