Allow for no-std build (extracts std feature, on by default)

[kevinclark]

The only things keeping rtp-rs from building on no-std are RtpPacketBuilder::build (build_into is fine), the two Error trait impls, some references to std that could be core, and some use of format! in error messages. After Rust 1.18 the errors can be no-std as well since std::error::Error is moving to core. But in the meantime I've got this branch that makes this usable, so I figured I'd share it and see if you're interested in supporting that usecase.

Tests, including doc tests, all still work, though I've moved them to use build_into except for a doc test that shows build still does it's thing. They both delegate the real work to build_into_unchecked so it seemed reasonable to me, but let me know if you feel differently.