Deprecate Async.fromCallable in favor Observable.fromCallable

[kboyarshinov]

Recently Observable.fromCallable operator was added to core package in 1.0.15(ReactiveX/RxJava#3154). It serves the same function as Async.fromCallable which means fromCallable in this package can be deprecated.

cc @akarnokd @artem-zinnatullin

[akarnokd]

Sure. Would you like to submit a PR?

[kboyarshinov]

Yep. I'll submit it.

[akarnokd]

Not necessary, #8 has it.

[kboyarshinov]

ok, cool!