implement eval function

summary:
"eval" is provided as a wrapper to eval_string (parse.h). In
principle, this could be achieved simply using feval("eval", but
the advantages to this implementation are:
1. faster (avoids double call and double conversion of code string)
2. explicit control of printing rather than implicitly with nargout (as in eval)
3. separate exception classes for parse error / execution error

thanks to David Grundberg for helping with preparing the patch