Support reading from and writing to memory

Hi,

ezodf lacks of such support while odfpy had it. I originally aimed at "ods" file type only but it turns out "odt", "odg" and "odp" are OK with my patches too.

The patches only modifies the behavior of file opening and file save. Although it causes back compatibility concerns because opendoc was inserted a file_content parameter, I don't think opening from a memory stream ever worked. Hence, it is pretty safe to add such an extra paramter. Test cases were appended. 

I am looking forward to its release in pypi as my library pyexcel-ods3 would like to have this support. 

Cheers

chfw