Added $slice projection operator

Returns a subset of N element from a stored array

Collection::where("field", "value")->slice("arrayField", 3)->get();

Implemented only for "Normal query"
changed array declaration to support PHP 5.3
Merge remote-tracking branch 'upstream/master'
Added basic $elemMatch support
Added $slice and $elemMatch unit tests

Returns the first element of a sub array matching given criteria. Support for =

User::where('id',1)->elemMatch('subArray', 'subArrayField', 'subArrayValue')->first();

Todo: implement all operators
array in PHP 5.3
Extended $elemMatch operators

Support all operators in where style, as well as `and` and `or`

```
$user = User::where('_id',1)
           ->elemMatch('mix', 'n', 3)
           ->orElemMatch('mix','n','>', 10)
           ->orElemMatch('mix','n','!=','x')
           ->first();
```
This will select the **first** element of `mix` subdocument where `n = 3 OR n > 10 OR n != x` applied to all `mix` subdocuments is true.

Not tested with multiple nesting levels.
Removed echo
Merge branch 'master' of https://github.com/disfasia/Laravel-MongoDB
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