a2_methodSignaturesCompatible only checks first character now.

I changed `a2_methodSignaturesCompatible` to only compare the first character of each argument type because there are new modern objc compilers out there which encode the whole expected objc class into a block argument. This means that the argument signature of the first argument of

```
^{UITextField *textField} {

};
```

will be `@"UITextField"` instead of a plain `@` which causes `a2_methodSignaturesCompatible` to fail.