Extend usage message for `swift download`

Make it clear that you do not have to provide either the container or
object positional parameters, and what swiftclient will do if you omit
them.

Change-Id: I3766fc1eabd51f197e2175771af579577a96c192
user defined headers added to swift post queries

This commit fixes a problem where headers passed through the option
--header were not taken into account when doing "post" queries.

Change-Id: Ia34808c53a1816d9956a98e0803d8c4d20d2ad44
Closes-Bug: #1226256
Merge "user defined headers added to swift post queries"
Merge "Extend usage message for `swift download`"