ENH: Add uint and bool support in to_stata
Added support for uint (uint8, uint16 and uint32, but not uint64) and bool
datatypes in to_stata.  
closes #7097 
closes #7365

we'll just call you mr. stata from now on :)

can you move the release note to 0.15.0.txt (0.14.1 coming tomorrow), so too late for that 
 I think this is ready.
 @jreback I added some docs.
 minor edits, otherwise looks good. 
 thanks @bashtage continued awesome stata support is great!
 merged via https://github.com/pydata/pandas/commit/7745d726b0625c8a4ab017629be0c4eb6241a81a
