to_geodataframe method for DataFrames
I often find myself converting from pandas `DataFrame`s to geopandas `GeoDataFrame`s, where I convert some position (from longitude/latitude, x/y, etc. columns) into a `Point` geometry column. It's easy and takes only a few lines, but why not summarize it along the lines of 
```
df.to_geodataframe(x=x_column_name, y=y_column_name)
```
(possibly extendable to other kinds of geometries as well)?

I'm happy to send a PR but I've never contributed before, so wanted to hear some feedback before doing potentially unwanted work.