use pycrypto to compute pub/priv rsa fingerprints

* Replace pyasn1 with PyCrypto as a direct dependency of StarCluster
* Deprecated SSHClient.{generate_rsa_key,get_public_key} in favor of
  sshutils.{generate_rsa_key,get_public_key}
cluster: validate imported keypair fingerprints
awsutils: add import_keypair function

Takes an existing RSA public key and uploads the public material to EC2
via the ImportKeyPair API.
awsutils: raise exc if keypair exists in create_keypair
awsutils: use 'as' everywhere when handling exceptions
commands: add _file_exists callback to CmdBase

The callback expands the path, converts it to an absolute path, and then
verifies that it 1) exists and 2) is a file.
createkey: add --import-key option

This option allows importing an existing RSA key as a new EC2 key rather
than having EC2 autogenerate and return a new RSA key.
Merge branch 'keypair-utils-improvements' into develop