Add isDefined method

The method _.isDefined is the good brother of _.isUndefined. IMHO it results in a more understandable code if you directly see what a check really mean. So i think its better to use 

```
if (_.isDefined(foo)) { .... }
```

instead of

```
if( false === _.isUndefined(foo)){ ... }
```

Both means **is something defined?** Well, it looks maybe a little obviously, but  could improve readability of code.