BUG: Adding nano offset raises TypeError
Closes #9284.

this looks good
ready to merge?
 Let me add explicit tests for backward compat and return type checks (pointed in #9309). I expect to finish it today.
 @jreback It's ready. Pls review.
 can u test a pickle from a previous version working?

you will have to use say 0.15 and run the test script to generate data 

you may need to modify the test script to pickle with a freq on the Timeseries

this is the generate_test_data in the pandas/io/tests/generate_legacy_pickles
 thanks @sinhrks 
