Named column variables for CSV

• Added support for 3 new variable notations (see #6):

1. attribute syntax, e.g. a.good_alphanumeric_column_name
2. dictionary syntax, e.g. a["arbitrary column name!"]
3. array syntax, e.g. a[3]

• Switched default encoding for CSV files to utf-8
• Implemented Join on NR, see (#10)
• Bug fixes