Use OOM lang item about embedded-alloc

[japaric]

Thanks for the report. With the new oom lang item the allocator doesn't have to define what happens on an Out Of Memory (OOM) condition. The behavior can now be specified by the application, or any other crate.

I think it's best to not define the oom lang item here and instead let the application decide how to deal with OOM.

alloc-cortex-m now fails to build

alloc-cortex-m builds just fine; it's the binary crate the one that errors if oom has not been defined.

I have updated the crate level documentation in #15 to show how to define oom.

[fmckeogh]

Thanks! :D