Make euclid build on stable rustc

[nical]

This PR adds an opt-in cargo feature "rustc_stable" to remove the subset of the library that doesn't build on stable rust.

[KiChjang]

src/side_offsets.rs:13:5: 13:8 error: unresolved import `num::Zero`. Maybe a missing `extern crate num`? [E0432]

src/side_offsets.rs:13 use num::Zero;

                           ^~~

src/side_offsets.rs:13:5: 13:8 help: run `rustc --explain E0432` to see a detailed explanation

error: aborting due to previous error

Build failed, waiting for other jobs to finish...

src/side_offsets.rs:13:5: 13:8 error: unresolved import `num::Zero`. Maybe a missing `extern crate num`? [E0432]

src/side_offsets.rs:13 use num::Zero;

                           ^~~

src/side_offsets.rs:13:5: 13:8 help: run `rustc --explain E0432` to see a detailed explanation

error: aborting due to previous error

Could not compile `euclid`.

[nical]

I'm installing multirust and will figure out the build failures on nightly (they are in the part that's stripped out when opting into stable). But you guys can already see the approach. Let me know if I am going in the wrong direction.

[metajack]

r? @mbrubeck

[kaksmet]

Other servo dependencies with a cargo feature to make use of nightly rust has their's named "unstable" (e.g. string-cache, tendril and heapsize).
I therefore think "rustc_stable" should have it's meaning inverted and be renamed to "unstable"?