add get method to avoid exception and set default

resouce.py causes `AttributeError` when I access to the missing attribute.

It is usual behavior for dictionary object, but number of attributes differ between tracks.
For example, one track has `comment-count` attribute, but the other is not ( id `149266904` track doesn't have comment-count).

Because of this, I have to check attributes's existence before access it, and it is a time‐consuming work.
So I added `get` method to safe access and to set default.