Fixes for issues #362 and #880: Unsigned integer table columns.

This pull request takes care of both #362 and #880.  

PS, I really wanted to convert #362 into a pull request, but both issue2pr.py & hub pull-request failed.

    weaver@Orac[68]: ~/Downloads/hub pull-request -i 362 -b astropy:master -h weaverba137:uint-scale
    github.com username: weaverba137
    github.com password for weaverba137 (never stored): 
    Error creating pull request: Unprocessable Entity (HTTP 422)
    Not allowed to change field "issue"

    weaver@Orac[4]: python issue2pr.py uint-scale 362
    Converting issue 362 of astropy/astropy to pull request
    Will request to pull branch uint-scale of user's astropy repo to branch master of astropy/astropy
    Username: weaverba137
    Password: 
    HTTP Error HTTP Error 422: Unprocessable Entity