Add getDefaultInstance method to StorageOptions and DatastoreOptions

[mziccard]

This follows from #342 and adds a static getDefaultInstance method to both StorageOptions and DatastoreOptions.

[ajkannan]

In v0.0.10, it looks like the method was called defaultInstance() in DatastoreOptions instead of getDefaultInstance(). However, in all other places in the API (RetryParams and ExceptionHandler), the method is called "getDefaultInstance", which is consistent with the method names chosen here. This looks good to me, but if @aozarov prefers to leave out the "get" in method names like this, then we should probably standardize for those two objects as well.

[mziccard]

@ajkannan I picked getDefaultInstance for consistency with RetryParams and ExceptionHandler. If we decide to remove the get part I'll take care of removing it everywhere.

[ajkannan]

Sounds good. And yes, the defaultInstance() was accidentally removed in 0.0.11; just for reference, here's a link to the method in the 0.0.10 javadoc.

[aozarov]

+1 for having it as defaultInstance(). I prefer it because it is shorter, consistent with its own file and overall more consistent with our style.

RetryParams and ExceptionHandler were copied and modified from another library I worked on before but I neglected to update it to conform our style. Would be great if we can take care of that as well...

[mziccard]

I renamed the methods to defaultInstance, updated the READMEs and made both RetryParams and ExceptionHandler conform with our code style.

[aozarov]

Great, thanks. Shall we change this file as well?

and replace DatastoreOptions.builder().build() with DatastoreOptions.defaultInstance()?

[mziccard]

Updated also site's landing page.