Make backtrace-sys optional

[vi]

Resolves #313.

[vi]

For this to work reliably, libraries (but not apps) depending on failure should use default-features=false, features=["std"].

Alternative is not to include default = [..., "backtrace/default"].

There can be a separate dummy crate like failure-backtrace that just enables backtrace/default, for app developers to cargo add easily. Or users may be instructed to depend on backtrace directly to get enable backtraces in failure.