Change transfer_summarizer to log on 10% boundaries

This change causes transfer_summarier to only log
at info level when percentage crosses 10%
boundaries, which is useful when uploading or
downloading very big files. Previously it would
log every percentage between 0% and 0.5%, then
between 9.5% and 10.5% and so on.

Fixes #169
Merge pull request #170 from snaury/master

Change transfer_summarizer to log on 10% boundaries