Adds human readable format for memory.

Resolves #27

* CLI: The display of memory in `cci details` and `cci list` with the 'table'
       format (default) will display humanized RAM sizes '1G', '2G', '3G'.
* CLI: Re-implements the interface we use from Pretty-table and makes a method
       that can return a PrettyTable from a Table instance. This decouples the
       library code from our use of it. The new way support alignment, sorting,
       formatting columns and formatting specific row entries.

Example of what this looks like:
```bash
$ sl cci detail --id 1425074
:............:.........................:
:       Name : Value                   :
:............:.........................:
:         id : 1234567                 :
:   hostname : aaaaaaaaaaaaa.sftlyr.ws :
:     status : Active                  :
:      state : Running                 :
: datacenter : sng01                   :
:      cores : 2                       :
:     memory : 4G                      :
:  public_ip : 119.81.40.110           :
: private_ip : 10.66.114.32            :
:         os : Ubuntu                  :
:............:.........................:


$ sl cci list
:.........:............:....................................:.......:........:................:...............:..............:
:    id   : datacenter :                host                : cores : memory :   primary_ip   :   backend_ip  : provisioning :
:.........:............:....................................:.......:........:................:...............:..............:
: 1276651 :   dal05    :      aaaaaaaaaaaaa.sftlyr.ws       :   4   :   4G   :  182.172.4.45  :  10.40.178.73 :              :
: 1389931 :   sjc01    :         bbbbbbb.sftlyr.ws          :   1   :   1G   : 103.168.233.71 :  10.52.41.155 :              :
: 1257021 :   sjc01    :   ccccccccccccccccccc.sftlyr.com   :   1   :   1G   : 106.168.233.35 :  10.54.83.221 :              :
:.........:............:....................................:.......:........:................:...............:..............:
```