Rename pskc.parse to pskc.xml

This renames the parse module to xml to better reflect the purpose of
the module and it's functions.

This also introduces a parse() function that wraps etree.parse().
Move encryption functions in pskc.crypto package

This moves the encryption functions under the pskc.crypto package to
more clearly separate it from the other code. Ideally this should be
replaced by third-party library code.