Show closest matching methods on MethodError

Implements my idea described in #7686 and #7512.

Screenshot what it looks like:
![bildschirmfoto vom 2014-07-24 13 21 01](https://cloud.githubusercontent.com/assets/80071/3687372/44335fbe-1326-11e4-9f0a-ec2c7daad058.png)
More concisely: `isa(i[1], i[2])`. Also might be clearer to use `for (arg, sigtype) in zip...` instead of referring to `i[1]` and `i[2]` here.