Add chart_type property

[fzumstein]

Add ability to change chart_type.

[davideroverso]

Just asked about this on stackoverflow, http://stackoverflow.com/questions/22855654/controlling-chart-type-in-xlwings

(did not have enough credits to create a new tag "xlwings")

[fzumstein]

I've posted a solution here until it's implemented: http://stackoverflow.com/a/22862213/918626

[fzumstein]

Closed by b179bd8