Want support for Role header (and non-default roles)

[arekinath]

Currently in MantaHttpHeaders we can set the Role-Tag header (using the confusingly named setRoles(), which aside: should really be setRoleTags() or at least document that that's what it does), but not the Role header, which we need to be able to set on the request in order to take up non-default roles.

This header is also important with GETs and other read-only operations, not just writes, so if we want to add it to MantaHttpHeaders, it would be nice if there were a few more function overloads to take it (e.g. MantaClient#get(String, MantaHttpHeaders), MantaClient#find(String, MantaHttpHeaders), MantaClient#listObjects(String, MantaHttpHeaders) etc)