fix xml elements namespace and sharedString/styles file search

[dorym]

This fixes 2 things:

• I encountered some spreadsheets that have namespaces in their xml tags.
  This sets the default lookup for any namespace.
• styles and shareStrings file paths are concatenated in a wrong way where is some
  cases the given path in workbook.xml.rels is absolute.
  I used the same solution in _parseTable(). It may be useful to extract this logic.

[sestegra]

Thank you for this PR.
Can you add a test file and a test case for it?

[sestegra]

@dorym To merge the PR, a test file and its related test case are required.