Convert to using symbols for column names.
Merge pull request #509 from tshort/symbol-column-names

Convert to using only symbols for column names.
No peeking (in readtable)
Add readnrows! methods for faster parsing
Merge pull request #513 from garborg/readtable

Speed up readtable
Add append function
Stop exporting some names
Infer separator from extension in readtable