Added wcs setter to ccddata.py

[stottsco]

Edit: Closes #203

[mwcraig]

@stottsco -- thanks for the pull request!

Two comments:

• Could you please add a test for your fix? Email me or wait til or regular meeting if you need help with that.
• I don't think you need to define the wcs property getter, just need the setter (one of the superclasses of CCDData already defines the getter). I might be wrong though...

To update your PR, make the changes locally, commit them, then push to your branch on github.

[bsipocz]

@stottsco - Could you please rebase this as travis is not stating up properly (due to an independent change in conda; this has been addressed in master since).

[mwcraig]

Superseded by #256