Allow SSHKey to be specified by fingerprint (Fixes #76)

This change modifies __get_ssh_keys_id to return a list containing IDs or fingerprints. The code assumes that fingerprints consist of 16 hexadecimal pairs separated by colons (e.g. 01:23:45:67:89:af:ca:ef:FE:DE:CB:A9:87:65:43:21).

This change also modifies SHHKeys.load. Allowing you to load a SHHKey by fingerprint.

examples:

    d = Droplet(...)
    d.create(ssh_keys=['01:23:45:67:89:ab:cd:ef:FE:DE:CB:A9:87:65:43:21'])

or

    key = SSHKeys(fingerprint='01:23:45:67:89:ab:cd:ef:FE:DE:CB:A9:87:65:43:21')
    key.load()
    d = Droplet(...)
    d.create(ssh_keys=[key])

I have not tested these modifications, but it should be fine to merge since it will not break any existing code (it might introduce buggy new features).