Squashed commits implementing format argument, sans io/connect.py update

* 95592e1 Fix multiple registration problem for aliased formats
* 0889189 Redo the auto I/O registration in a more consistent way
* 178a2e4 Add table showing available ASCII I/O connector formats
* 331a69f Add auto-registration of readers/writers  (PARTIAL)
* d0841b1 Add format keyword to ascii.read and write methods
Update io.ascii.connect to provide functions for generic reader registration
Fix mistake in setting global FORMAT_CLASSES
Move info about IPAC position keyword into docstring
Using ascii. instead of ascii: to prefix ascii subformats
Shorten _description for fixedwidth_two_line slightly
Define AAStex io_registry_suffix to be '' to prevent .tex suffix from Latex
Rework table/io docs to reflect new ascii register scheme
Add tests of read/write with format arg
Refactor docs to remove redundancy and use format arg everywhere
Fix python 2.6 incompatibility in test_write
Fix doc ref mistake and provide additional refs for read/write params
Fix python 2.6 incompatibility in test_read
Merge pull request #961 from taldcroft/ascii-format-arg

Add format arg to io.ascii read/write and overhaul io.ascii.connect