Replaced nasty eval() code.

I have removed the eval() calls by using a custom eval_expression() method that parses the given statement without using eval().
This is especially important if you are using PHP in a SUHOSIN environment where the eval() function has been disabled. Using this (quite common) web server configuration you can in fact not use roundcube mail without this fix.

PS: I have tested the fix and I am already using this fixed code for my own mail server.

Regards