- implemented key protection such that unprotected private keys can be protected with passphrases, and already-protected passphrases can be re-protected with a new passphrase, and unit tests for these two scenarios - closes #149

- updated CI configs so that Python 3.2 can actually finish running the test suite without crashing since it is stuck on an older version of Cryptography
- added additional tests to test setup.py on all versions of Python after I noticed that a recent change would have made it break on Python 3.2
- enum34 should no longer be installed needlessly on Python 3.4 or newer
- added --no-cache-dir to tox's call to pip to install dependencies needed for testing

tox.sh

@@ -2,12 +2,9 @@
 
 # homebrew is installed and so is a brewed openssl
 if [[ $(uname) == "Darwin" ]] && command -v brew &>/dev/null && brew list openssl &>/dev/null; then
-    rm -rf ~/Library/Caches/pip/*
-
-#    env ARCHFLAGS="-arch x86_64" LDFLAGS="-L/usr/local/opt/openssl/lib" CFLAGS="-I/usr/local/opt/openssl/include" tox $*
     export ARCHFLAGS="-arch x86_64"
     export LDFLAGS="-L/usr/local/opt/openssl/lib"
     export CFLAGS="-I/usr/local/opt/openssl/include"
 fi
 
-tox $*
+tox $*