BUG: GH3493 fix Cannot append DataFrames with uint dtypes to HDFStore
Fix for self-reported uint bug in HDFStore. #3493
Travis results (pass): https://travis-ci.org/jmellen/pandas/builds/6764554

pls add a mention in RELEASE.rst referencing the bug and issue
 Sorry for the delay this afternoon; here's the release note update.  Thanks!
 thanks for the PR!
