Simplify DataType value handling

Only store the native value of the property, not the text
representation. This also results in the BinaryDataType and
IntegerDataType subclasses only needing from_text() and from_bin()
functions.
Introduce mk_elem() to create elements

This introduces the mk_elem() function that can be used to create
ElementTree elements for building XML documents. This function
transparetly handles namespaces, translation of values into XML etc.
Construct XML document with basic PKSC information

This introduces make_xml() functions to build an XML document that
contains the basic PSKC information and keys. This currently only
supports writing unencrypted PSKC files.
Add function for writing XML

This provides a function for pretty-printing the generated XML document.
Add test for writing PSKC files

This makes a simple doctest that checks the writing of the XML
representation of the PSKC data.
Support writing unencrypted PSKC files