feature: Document.headers and footers #104

def get_headers(file):
    import xml.etree.ElementTree as ET
    document = Document(file)
    namespace = dict(w="http://schemas.openxmlformats.org/wordprocessingml/2006/main")
    header_uri = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/header"
    headers = []
    for rel, val in document._document_part._rels.iteritems():
        if val._reltype == header_uri:
            xml = val._target._blob
            root = ET.fromstring(xml)
            text_element = root.find(".//w:t", namespace)
            if text_element.text is not None:
                headers.append(text_element.text)
    return headers