numerous REPL fixes

Hi Gravicappa,

This pull request fixes the following bugs:

* feeding REPL with empty string throws error
* feeding REPL with symbol prints weird symbols
* (value undefined*symbol) gives weird results (should give 'undefined')
* partial functions throws error

The following 2 files are changed:

`shen-node.js`, `runtime-node/lib.js`: adds checks for invalid values

PS: there is a bug when calling `shenstr()` with undefined values that gives weird symbols.

Regards,

xkxx