Expand test coverage

More tests! Adds tests for Droplet and Domain classes as well as fixes a few issues that were exposed by the testing.

Up to 80% coverage:

```
$ nosetests --with-coverage --cover-package=digitalocean
.......................................
Name                   Stmts   Miss  Cover   Missing
----------------------------------------------------
digitalocean              17      0   100%   
digitalocean.Action       32      1    97%   51
digitalocean.Domain       44      7    84%   20-22, 67, 70, 73, 118
digitalocean.Droplet     186     27    85%   98-100, 105-107, 131, 253, 329-349, 361, 365, 404, 472
digitalocean.Image        31     12    61%   23-25, 28-35, 41, 50, 60, 67
digitalocean.Kernel        9      1    89%   12
digitalocean.Manager     118     17    86%   24, 37-46, 76, 88, 118, 162, 180, 186, 189
digitalocean.Record       36     17    53%   23-25, 31-47, 53, 62-70, 77-84, 87
digitalocean.Region       11      1    91%   14
digitalocean.SSHKey       41     25    39%   19-21, 24-34, 43-49, 55-67, 73-85, 91, 97
digitalocean.Size         14      1    93%   18
digitalocean.baseapi      57      8    86%   5-6, 40-41, 55, 64, 88, 91
----------------------------------------------------
TOTAL                    596    117    80%   
----------------------------------------------------------------------
Ran 39 tests in 0.087s

OK
```