implement gpg.EncryptedFile

EncryptedFile allows to transparently interact with an encrypted file.
The class implements a non-streaming interpretation of traditional
filelike-semantics, because streaming would be more complex to implement
and we don't need it for now.
test that EncryptedFile expands the provided filename
provide more descriptive names for some tests

I usually avoid to invent long function names for the sake of brevity,
but for the nose output with have to make up for missing context.