to_stata uint16
Simple changes to io/stata.py to write unsigned integers to Stata files. Sorry about the poor commit messages--I'm still new to git and wasn't able to correct it cleanly.

(See issue #7365)

you need to add some tests in io/tests/test_stata.py

you create a frame with several dtypes then round trip and compare with the original

these tests should fail (with uint16 dtypes) before your fix and pass after
 I'm not sure this is a good idea since Stata doesn't support any unsigned data types.  In particular some of the code which uses the numpy datatypes in this commit will result in data loss.  

The only place where uint\* handling should be added is in the casting code.  Once the columns have been cast to supported Stata datatypes then only the supported Stata datatypes should be used when writing the Stata data types (e.g. `char(253)`)
 Also noticed a clear bug in (my) the code

https://github.com/dmsul/pandas/blob/tostata-uint16/pandas/io/stata.py#L244

should be 

```
if data[col].max() <= 2 ** 53 or data[col].min() >= -2 ** 53:
```

The first comparison has `*`, not `**`.  Would be good to fix this.
 @dmsul ok, then this needs to be changed to coerce `uint` types to `int` (and if they are out of range, then I would raise (e.g. a >`int64` value from a `uint64`)
