r_toolbox: Update

Hi folks,

I have updated the functions of the r_toolbox.R set of utilities:

https://sourceforge.net/projects/r-project-utilities/files/?source=navbar

Naming was changed with some functions to reflect similar functions in SAS 
or SPSS, e. g. t_n_miss, t_n_valid. In addition I added functions for 
reporting memory usage, selecting variables by type and getting an 
overview over the levels of factors.

I hope you find these functions useful.

Please get back to me if you have suggestions or encounter any 
difficulties.

Kind regards

Georg