Auto merge of #2396 - sbeckeriv:network-retry-1602, r=alexcrichton

Network retry issue 1602

Dearest Reviewer,

This branch resolves #1602 which relates to retrying network
issues automatically. There is a new utility helper for retrying
any network call.

There is a new config called net.retry value in the .cargo.config
file. The default value is 2. The documentation has also been
updated to reflect the new value.

Thanks
Becker