Support for multiple t nodes in strings

Sometimes formatting (specially with japanese characters) separates text in multiple t nodes. Just "get" will result in only one node. Which works in many cases but breaks in some.
Ignore phonetic text nodes

Japanese Excel occasionally stores phonetics as well as the regular text in the xlsx sheet. Those nodes do not show up to a excel user so they can be ignored.
Merge pull request #25 from martinheidegger/patch-1

Support for multiple t nodes in strings