Add SNS support

Adds SNS support. Full feature set from README:

Short Notification Service (SNS) is Amazon's technology that allows you to be notified when actions are completed. `glacier-cmd` allows for whatever granularity of control as you desire.

If you run `glacier-cmd sns sync` without specifing anything in your configuration file, it will automatically subscribe all your vaults to `aws-glacier-notifications` topic.

  $ glacier.py sns sync                                           
  +------------+-------------------------------------------------+
  | Vault Name |                    Request Id                   |
  +------------+-------------------------------------------------+
  |  vault1    | r6egKGUGtCPFi0uEQwP9hIcl5TRSr_EhxxogfX56RnN9FxA |
  |  vault2    | bXxvIX-gaALibuU7OHOL8OIy1EAPgDDknnswV8DlOsESMAI |
  |  vault3    | iJTMya-QvQ5Jf17hMd85vY9qk1q2dNwnR90aS-p5Vwl__PY |
  +------------+-------------------------------------------------+

However, if you desire, you can put sections into your settings file to allow for finer control of your notifications. 

  [SNS:topic1]
  method=email,valid.email@address.com;

  [SNS:topic2]
  vaults=vault1,vault2
  method=email,valid.email-1@address.com;email,valid.email-2@address.com

Topic name is specified after `SNS:`. By default, if you don't specify any vaults, all your vault will be subscribed to that topic. 

If you pass in the method argument you will be automatically subscribed to that topic.

After any changes to your `.glacier-cmd` file you should run `glacier-cmd sns sync` again.

  $ glacier-cmd sns sync
  +--------+----------------------+------------+-------------------------------------------------+
  | Topic  |   Subscribe Result   | Vault Name |                    Request Id                   |
  +--------+----------------------+------------+-------------------------------------------------+
  | topic1 | pending confirmation |  vault1    | Ap-i98165PUh8eSnppwdstANnwSkcOe9VAqXEhIyY45ybYc |
  |        |                      |  vault2    | QJOigsIM_JPKG1SEhMYmkbxZzxhQ9lkN4HLKLJKuqaKDHg0 |
  |        |                      |  vault3    | 5cXDpeL91-ZKNIDKs5nUzqIJ9O1ktujwcPQtQf2tLHmgr54 |
  | topic2 | pending confirmation |  vault1    | y8mmcnWrK5R5nQzmz5sxlJKD20DVkyDkUkOywo8p273TUDA |
  |        |                      |  vault4    | gvrFKzO7W4Srr8NoLQLnCxuw64vf_7qpLSdAe_6Pon7KqfQ |
  +--------+----------------------+------------+-------------------------------------------------+

To get a list of all subscriptions you can run `glacier-cmd sns lssub`.
  
  $ glacier-cmd sns lssub                                      
  +--------------+-----------+----------+-------------------------+-----------------------------------------------------------------------------------+
  |  Account #   |  Section  | Protocol |         Endpoint        |                                        ARN                                        |
  +--------------+-----------+----------+-------------------------+-----------------------------------------------------------------------------------+
  | 123456789101 |   vault1  |  email   | valid.email@address.com |                                PendingConfirmation                                |
  | 123456789101 | allvaults |  email   | valid.email@address.com | arn:aws:sns:us-east-1:123456789101:allvaults:xxxxxxxxxxxxxxxxxx                   |
  | 123456789101 |  vault2   |  email   | valid.email@address.com |    arn:aws:sns:us-east-1:123456789101:vault2:xxxxxxxxxxxxxxxxxx                   |
  | 123456789101 |   bleble  |  email   |     mail@example.com    |   arn:aws:sns:us-east-1:123456789101:bleble:xxxxxxxxxxxxxxxxxx                    |
  | 123456789101 |   bleble  |  email   | valid.email@address.com |                                PendingConfirmation                                |
  +--------------+-----------+----------+-------------------------+-----------------------------------------------------------------------------------+

To get a list of all topics (including non-glacier-related ones) run `glacier-cmd sns lstopic`.

  $ glacier-cmd sns lstopic
  +-------------------------------+------------------------------------------------------------------+
  |             Topic             |                            Topic ARN                             |
  +-------------------------------+------------------------------------------------------------------+
  |   aws-glacier-notifications   |   arn:aws:sns:us-east-1:123456789101:aws-glacier-notifications   |
  |            vault2             |             arn:aws:sns:us-east-1:123456789101:vault2            |
  |            vault1             |             arn:aws:sns:us-east-1:123456789101:vault1            |
  +-------------------------------+------------------------------------------------------------------+

To subscribe to a specific topic, run
  
  $ glacier-cmd sns subscribe protocol endpoint topic
  +----------------------+--------------------------------------+
  |   SubscribeResult    |              RequestId               |
  +----------------------+--------------------------------------+
  | pending confirmation | xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx |
  +----------------------+--------------------------------------+

You can also pass in the `--vault` argument which will subscribe all specified vaults (separated by comma) to a specified topic.

  $ glacier-cmd sns subscribe email valid.email@address.com topic10 --vault vault1,vault2,vault3
  +----------------------+--------------------------------------+
  |   SubscribeResult    |              RequestId               |
  +----------------------+--------------------------------------+
  | pending confirmation | e9a1d360-193c-52e4-89b5-9a78f08252ea |
  +----------------------+--------------------------------------+

To unsubscribe, use unsubsribe, which you can limit with --protocol, --endpoint and --topic.

  $ glacier-cmd sns unsubscribe --endpoint valid.email@address.com --protocol email --topic vault2
  +--------------+---------+----------+-------------------------+-----------------------------------------------------------------------------+
  |  Account #   | Section | Protocol |         Endpoint        |                                     ARN                                     |
  +--------------+---------+----------+-------------------------+-----------------------------------------------------------------------------+
  | 123456789101 |  vault2 |  email   | valid.email@address.com | arn:aws:sns:us-east-1:123456789101:vault2:xxxxxxxxxxxxxxxxxxxxx             |
  +--------------+---------+----------+-------------------------+-----------------------------------------------------------------------------+

You have to pass in at least one option. If you pass in only one option, all subscriptions matching that option will be unsubscribed. So if you would pass in `--endpoint valid.email@address.com` all subscriptions to that address would be unsubscribed.