make prompt setting more robust against roque and slow shells
improve up ec2 example
improve consistency on how and when contexts are initialized
make deep_copy behave more sensible on private attributes (which are not copied)
add an existence check
clean ec2 example
additional robustness for  context evaluation

ssh and ec2_keypair contexts are now more robust on key specifications -- they
both support user_key and user_cert (the latter for backward compatibility), and
accept public and private keys (the complete keypair is derived from the
respectively specified key).