Issue 251. Pulled bindPlayServices() out of constructor and into separate initialize() method

[autonomousapps]

Code is self-explanatory, but breaking for current users.

Rationale:
Sometimes, bindPlayServices() finishes its asynchronous work before the constructor finishes. This can mean that onBillingInitialized() is called before the constructor finishes, and so work involving a billingProcessor can result in NPEs.

[serggl]

@autonomousapps this should be done as an alternative constructor option I guess

[autonomousapps]

Ah, that's a good idea. How do you feel about static factory constructors? Or would you prefer another constructor with, say, a boolean parameter?

[autonomousapps]

I've updated my PR to add overloaded constructors and factory methods so that this change will be non-breaking to current library users.

[serggl]

@autonomousapps this now looks good. Can you please update README with new factory method description/instructions ?

[autonomousapps]

Absolutely. I'll have it updated today.

I see the build failed with:

Checkstyle rule violations were found. See the report at: file:///home/travis/build/anjlab/android-inapp-billing-v3/library/build/reports/checkstyle/checkstyle.html

I haven't used TravisCI, so I'm not sure how to see the style failures. Could you tell me if they're my new code and how I can address them?

[autonomousapps]

Updated PR and squashed into one commit.

[serggl]

@autonomousapps sorry my fault. I should probably update readme with a checkstyle requirements.

Basically you need to ensure that ./gradlew checkstyle does not show any errors/warnings.

And ready to merge this PR once you will pass checkstyle. Thank you!

[autonomousapps]

I've amended my commit and force-pushed. Fixes all checkstyle issues.