increased verbosity in table.write/table.read errors (fixes issue #590)
This is a rebase / redo based on the IO refactor.  Haven't yet tested
whether _readers and _writers are the right variables; travis will
tell...
Adds some tests that caught the bug in #590 and fixed said bug.  Factored out some common functionality into a _get_valid_format internal function, and did a bit of pep-8 cleanup while I was in there.
Fix formatting issue of docstring
Merge branch 'verbose_tablewrite_exception'