Rename ValueTuple to ReadingTuple for consistency
Docstring fixup as per PR37
Replace all occurrences of value_tuple -> reading

Since these two terms refer to the same (timestamp, status, value) tuple,
pick the latest and slightly more descriptive one and make the lot consistent.
More value_tuple -> reading conversions
Fix socket exception handling on Mac OS X
Merge pull request #38 from ska-sa/Address-PR37-comments

Rename ValueTuple to Reading for consistency