Replace evpy crypto calls in rsa_key.py with PyCrypto's equivalent
Update rsa_key.py and keystore.py with final PyCrypto major changes

rsa_key.py was modified to generate and verify RSASSA-PSS signatures instead of RSASSA-PKCS1-v1_5.  Optional functions to read and save passphrase-protected PEM files also added to rsa_key.py.  keystore.py was modified to generate encrypted .key files (similar scheme as before) with PyCrypto to support uniform encryption of varied key types.  User passwords are no longer temporarily stored, but used to derive a symmetric key with PBKDF2.  The derived key is then used with AES-256-Mode-CTR to generate the encrypted key data.  Affected unit tests updated.
Merge branch 'develop' into pycrypto
Add tests for create_encrypted_pem() and create_from_encrypted_pem()
Continue updating unit tests affected by PyCrypto changes
Fix bug in test_signercli.py and update test case following PyCrypto changes

A bug in test_signercli.py prevents required keys from loading.  A password for a test keyid is modified but not properly restored.  This particular bug made it difficulty in adding a new feature (i.e., derived keys) and updating the unit tests.  We need to simplify the unit tests, specifically the removal of side effects, monkey patches, pseudo repositories/data structures, and dependency on scripts that expect user input.  'signerlib.py' should be called instead of the signer tools (e.g., signercli.py).
Set a lower PBKDF count for some of the unit tests

The default tuf.keystore._PBKDF_ITERATIONS (90,510) slow down the unit tests considerably.  Reduce it temporarily for the unit tests depending on it and that do not test the strength of derived keys.  All 164 tests now run in approximately 2 minutes, down from approximately 17 minutes.  The lowered PBKDF count set for the unit tests is equal to the previous key derivation count used by evpy.
Merge branch 'demo2' of https://github.com/theupdateframework/tuf into demo2
Merge remote-tracking branch 'origin/demo2' into demo2