ENH: adds ability to generate bq schema from df
I need this function for some other work I have in a branch (to_gbq w/ a new table and to_gbq with staging in gcs).  I saw issue #8325 and thought this might be a good intermediate step. 

seems reasonable. Maybe put in a doc mention in io.rst (gbq section)?

and a release note in v0.15.2 (referencing that issue)
 Ok, made the updates, hopefully all goes well with travis.
 @tshauck thanks!
