Support Fit File Upload

[jrast]

In jaroslawhartman/withings-sync#132 I implemented the upload of *.fit files. Any interest to move this code to garth?

[matin]

Let me take a look. I'm sure it'd also be useful to others.

[matin]

@jrast could you give #9 a try?

I wanted to find a to make it generalized.

You can do the following:

with open(fpath) as f:
    garth.client.upload(f)

[jrast]

Of course, If #9 is working go ahead and close this PR in favor of #9.
I will check if #9 is working but I will need some days (probably next week).