Simplify behavior of Keyword class

In response to comments by @embray
Clarify handling of units
Explicitly state that units are required for initialization
Add detail to i/o portion of api
Pass read keywords to FITS reader
User may not disable scaling when reading data from a FITS file.
Describe method for converting CCDData object to an hdu.

This was called toFITS in the initial draft of the CCDData code though it was not mentioned in earlier drafts of the API
Simplify handling of keywords that are strings
Fix typo
Merge pull request #5 from mwcraig/ccdproc

Update API to reflect github and offline discussion