Add function to encrypt environment variable keypair

[leeper]

Some notes:

# need a function here to do encryption and, optionally, add encrypted env var to .travis.yml
#x <- get_encryption_key("cloudyr/travisci")$key
#x <- readLines(textConnection(x))
#k <- PKI.load.key(base64decode(paste0(x[-c(1,length(x)-1)],collapse="\n")))
#base64encode(PKI.encrypt(charToRaw("a=b"), k))

How travis does this:

• https://github.com/travis-ci/travis.rb/blob/master/lib/travis/cli/encrypt.rb
• https://github.com/travis-ci/travis.rb/blob/master/lib/travis/client/repository.rb
• https://github.com/travis-ci/travis.rb/blob/master/lib/travis/tools/ssl_key.rb

[leeper]

Actually, Hadley already does this here. I can import from there or copy the code.