Small improvement to some exception messages
Add support for parsing the 'string' datashape as proposed

In doing so, I've moved some of the datashape construction
into the PLY semantic actions, in particular for the 'appl_args'
production rules. I think it might make sense to move all of
the build_ds functionality there, to make it easier for
people to see what's going on.
Fix how the String datashape object converts to a string
Fix out of bounds array access on end-of-string parse error
Add the ability to name datashape record fields 'type', 'int32', etc
Fix the line number/pointer in the datashape parse syntax error
Merge pull request #41 from mwiebe/string_dshape

Add string dshape support