Fix error in documentation of graph.ibars
Fix problem when writing CSV files with NA values
Keep windows open when invoked in non-interactive mode
Implement a general purpose quicksort algorithm

The old implementations of quicksort are removed.
Use wrappers in rnd functions to check for nil values
Use opaque type for gdt_table declaration

Hide implementation details from the C API.
Avoid exposing unsafe gdt_element with NaN encoding to the external
world.
Move gdt_table_get index checking in Lua code

More coherent with other functions like gdt_table_set_*.
Fix problem with integer overflow in gdt_table_new
Check early if gdt table creation fails
Add function to add new rows to a gdt table

Renamed insert/append column's methods to avoid conflicts.
Implement heapsort sorting algorithm
Improve quicksort by using heapsort if depth is exceeded
Implement more plotting options in gdt.xyplot
Added complex.arg(z)