refactor: move methods to client class

[HemangChothani]

Fixes#38

[andrewsg]

LGTM

[andrewsg]

@tswast In the bug you said, "we need to update samples for moved methods before deprecating the old ones." Is this change okay as-is, or do we need to make this change without the deprecation warning first, then update samples, then add the deprecation warning?

[tswast]

If it helps you find the samples that need to be updated, I'm okay merging this PR first. It's just a bad look if we have samples on cloud.google.com that show deprecation warnings.

[andrewsg]

I agree. But we can't update the sample without pushing part of this change, so @HemangChothani Let's omit the deprecation warning for now and open an issue to update the samples, then add the deprecation warning in that order.

[HemangChothani]

@andrewsg Opend an issue GoogleCloudPlatform/python-docs-samples#5031 to update the samples.

[HemangChothani]

@andrewsg I have opend an issue to update sample GoogleCloudPlatform/python-docs-samples#5031 in python-docs-sample repo , but i didn't find any sample needs to update, should i open issue in python-storage repo to update snippets and system-tests for list_blobs and download_to_file methods

[frankyn]

@andrewsg and @HemangChothani, I replied to the issue. We are only missing updates for download_to_filename.

[HemangChothani]

@frankyn we have moved download_to_file into the client class, download_to_filename is still in the blob class so i think there is no need to update any sample.

blob.download_to_filename is calling blob.download_to_file method which is deprecated now, so i will update soon and direct call the method client.download_to_file method.

Also update the method call from bucket.list_blobs to client.list_blobs.

[frankyn]

Why was only download_to_file updated but not download_to_filename?

[HemangChothani]

Document mentioned in issue #38, found download_to_file and list_blobs methods need to move implementation from blob class to client class and deprecate older one.