Allow custom headers when using swift download (CLI)

This fixes bug #1051046

A repeatable option, --header or -H, is added so a user can specify
custom headers such as Range or If-Modified-Since when downloading
an object with the swift CLI.

Change-Id: I1f7dcf64cf625f2e5a4488c210894cfe6e0d5974
Merge "Allow custom headers when using swift download (CLI)"