Fix build on 2015-01-28 nightly

[aroben]

See rust-lang/rust#21248.

[aroben]

Looks like there are still warnings during cargo test.

src/lib.rs:28:1: 28:19 warning: use of unstable library feature 'test'
src/lib.rs:28 extern crate test;
              ^~~~~~~~~~~~~~~~~~
src/lib.rs:28:1: 28:19 help: add #![feature(test)] to the crate attributes to silence this warning
src/lib.rs:28 extern crate test;
              ^~~~~~~~~~~~~~~~~~
src/atom/mod.rs:472:13: 472:26 warning: use of unstable library feature 'std_misc': may change with specifics of new Send semantics
src/atom/mod.rs:472             Thread::spawn(move || {
                                ^~~~~~~~~~~~~
src/atom/mod.rs:472:13: 472:26 help: add #![feature(std_misc)] to the crate attributes to silence this warning
src/atom/mod.rs:472             Thread::spawn(move || {
                                ^~~~~~~~~~~~~

But I can't just add #![feature(test)] unconditionally, because then it will fail to build in non-test configurations. I'm not sure how to make an attribute conditional.

[aroben]

Figured it out.

[kmcallister]

Rolled this into #61.