Remove magic from parse function

[stchris]

Right now the parse function "magically" detects if the passed input is a string containing XML, an URL or a filename. While this seemed convenient at the time it might cause some issues.

Proposal:

• add parse_file which takes a path
• add parse_xml_string which takes an XML string
• add parse_url which takes an URL

The parse function could be marked as deprecated and removed in version 2.0 ?