Use Symbols as column names

fixes #134

Should work as before when passing strings as column names: they're cast as symbols.

New to Julia so comments/thoughts/pointers welcome.

*Now I think about it maybe should also check identifiers (again inline with DataFrames), but I don't think there is a Base implementation of that yet?...*
Maybe this should be dried out. `string(ta1.colnames[1])[1:2]` appears a few times here.