Make error_chain with default-features = false work with Rust 1.10

[joshtriplett]

error_chain uses attributes on statements, stablized in Rust 1.13.
Factor them out of functions in the non-backtrace codepath, making it
possible to build a crate that uses error_chain with Rust 1.10.

The backtrace feature still requires Rust 1.13 for attributes on
statements, due to the impl_extract_backtrace macro, which needs to emit
a function with one block for each linked error type, some of which may
have attributes on them. And any code using compile-time conditionals
in attributes on variants within an error_chain! invocation (which
includes many of the tests and examples) will produce an error due to
rust-lang/rust#22250 (fixed in Rust 1.11).
But a crate using error_chain with default-features = false and avoiding
such compile-time conditionals will now compile.

[joshtriplett]

If it's not too much trouble, would you mind putting out an 0.7.2 with this included, so I can use that from another crate?

[Yamakaky]

That's better than nothing. Could you update the README?

[joshtriplett]

@Yamakaky Sure, will do. I'll also try to make Travis build-test the bits that work on 1.10.

[joshtriplett]

@Yamakaky Done.

The Travis bits aren't perfect, since they still declare two builds for 1.10 and skip one at build time, but they do work correctly. Working on that.

[joshtriplett]

OK, this now tests 1.10 properly with Travis.

[Yamakaky]

Thanks!