Support contexts

[arnaud-lb]

Implements #32

At the API level, this adds:

• BackOffContext interface: A BackOff that also supports contexts
• WithContext(BackOff, context.Context): Returns a BackOffContext from a BackOff

Retry() and Ticker still accept BackOff implementations, but also support BackOffContext implementations.

[coveralls]

Coverage increased (+0.5%) to 82.051% when pulling eff3c63 on arnaud-lb:context into b02f2bb on cenkalti:master.

[arnaud-lb]

This also breaks the compatibility with older Go versions which don't have the context package.

If you want to maintain the compatibility, we should be able to support older Go versions by declaring a backoff.Context interface, and accepting this interface instead of context.Context:

type Context interface {
 	Deadline() (deadline time.Time, ok bool)
 	Done() <-chan struct{}
 	Err() error
 	Value(key interface{}) interface{}
 }

[coveralls]

Coverage increased (+0.5%) to 82.051% when pulling 376009d on arnaud-lb:context into b02f2bb on cenkalti:master.

[coveralls]

Coverage increased (+0.02%) to 81.529% when pulling 4af6cef on arnaud-lb:context into b02f2bb on cenkalti:master.

[arnaud-lb]

Added support for older Go versions:.

This depends on "golang.org/x/net/context" instead of "context", as the former is always available.

This makes the package compatible with all Go versions supported by "golang.org/x/net/context".

It's still compatible with both "context" and "golang.org/x/net/context" packages.

[coveralls]

Coverage increased (+0.02%) to 81.529% when pulling 997c21a on arnaud-lb:context into b02f2bb on cenkalti:master.

[coveralls]

Coverage increased (+0.02%) to 81.529% when pulling 997c21a on arnaud-lb:context into b02f2bb on cenkalti:master.

[cenkalti]

@arnaud-lb Sorry for the delay, I was quite busy this week. I still couldn't review this. I will do it as soon as possible.

[metalmatze]

Is there any particular reason not to use the, in Go 1.7 introduced, context from the stdlib?
Do you want to be backwards compatible to 1.3, as it's defined in .travis.yml?

[cenkalti]

@metalmatze see #40