Numeric functions - Date Functions and TypeFromStringConverter

Hi!

I added the Numeric functions and Date Functions that are listed in the [new docs](http://docs.arangodb.org/Aql/Basics.html)

I also added a [TypeDescriptorConverter](https://github.com/jjchiw/ArangoDB-NET/blob/numeric-functions/src/Arango/Arango.Client/API/Documents/Document.cs#L1872) I think is more safe that `System.Convert.ChangeType(fieldValue, propertyInfo.PropertyType)`

I added that because I had a Guid in my POCO and when I tried to read from the database and deserialized it the `ChangeType` failed with an exception but the TypeDescriptor works flawless :)

I'm really liking ArangoDB.

I forked this project [NerdDinner with NacyFx and RavenDB](https://github.com/NancyFx/DinnerParty) so instead of use RavenDB as the database use ArangoDB and here is the fork https://github.com/jjchiw/DinnerParty/tree/arango 

And here it is how I used ArangoDB https://github.com/jjchiw/DinnerParty/tree/arango/src/Data

Thanks for the driver