enable setting multiple SAS permissions at once

[bmc-msft]

As indicated in the Specifying permissions,

Permissions can be combined to permit a client to perform multiple operations with the same SAS. When you construct the SAS, you must include permissions in the following order:

racwdxltmeop

Examples of valid permissions settings for a container include rw, rd, rl, wd, wl, and rl. Examples of invalid settings include wr, dr, lr, and dw. Specifying a permission designation more than once is not permitted.

Fixes #431.

Note, we probably want to create multiple SasPermissions style classes, as there are different sets of allowed permissions for different resources.