Add optional zs parameter to _transform, see #199

The function signature mimics pyproj:transform so zs is optional and can be set to None, if it is not set it is assumed to be zero and only res_xs and res_ys are returned.
Add optional zs parameter to transform, see #199

Sync API to df2ff9083c8fcad812ae81ea35a1ce1adb5fa273
Add tests for optional zs in warp.py:transform #199
Update wrap.py:transform docstring to 3D
Merge pull request #200 from alexamici/master

Add optional zs parameter to warp.py:transform, see #199