removed length from eval code to make it work with vdebug

This is based on the following patch:

https://gist.github.com/joonty/3348076

My intention was to make eval functionality in vdebug work with pydbgp. Since vdebug also works with other languages/debug hosts and only python gets special mention for possibly needing a patch, I assumed that vdebug is right (or at least more general) and pydbgp needs to be fixed. Also, the xdebug specification contains this entry:

> 2004-02-20
> [...]
> * 8.3 remove the length argument in the eval command, it is unnecessary.

http://xdebug.org/docs-dbgp.php#a-changelog

See also the eval spec:

http://xdebug.org/docs-dbgp.php#eval