ENH: Adds gcs module for IO with Google Cloud Storage
Hi,

This isn't ready for primetime yet, but I wanted to see if there was feedback, since it would be a new io module.

Basic usage:

```
import pandas.io.gcs as gcs
import pandas as pd
import numpy as np

df = pd.DataFrame({
    'a': np.random.normal(size=1e4),
    'b': np.random.normal(size=1e4)
})

gcs.to_gcs(df, "th-code", "pandas2.csv", "eighth-physics-623", to_gcs_kwargs={'resumable': True}, to_csv_kwargs={'index': False})
i = gcs.from_gcs("th-code", "pandas2.csv", "eighth-physics-623")
```

This borrows some of the connection patterns from `gbq`, though it can do service account auth in addition to OAuth2WebServer auth.  Might be nice at some point to centralize google api connection stuff.  If this is accepted the code to connect to google's APIs will be in 3 places.

I'm not sure everything this would need to be accepted, besides tests...

for sure want to fix the connection API / testing issues (that is a separate / distinct PR).

this seems like a seemless-file-transfer mechanism? can you post a link to the service
 You mean like: https://cloud.google.com/storage/?
 yep. You are moving a csv to the storage? (or a pickle?) do they have an API for it? (the format I mean).
 Yeah, right now I have it defaulting to `text/csv`.  Through the API it comes back from - and goes to - GCS as a string, so I'm using `StringIO` to go between. 

I have looked at other formats.  I believe it's possible to change the format by setting the `contentEncoding` parameter when inserting the object.  To me gzip is more appealing than pickle, but both would be nice.
 closing, pls reopen if/when this is ready
