Add serde default attribute for fields
Add serde default attribute for fields

When annotated with #[serde(default)], a field gets a default value
using std:default::Default, and will not error when decoding if the
field is missing from the input.

Addresses #9

sidred10/serde,eefriedman/serde,Ticki/serde,csmulhern/serde,pcwalton/serde,erickt/serde,JohnHeitmann/serde,tempbottle/serde,serde-rs/serde,Byron/serde,skade/serde,oli-obk/rust-serde,thorbenk/serde,infinityb/serde,jmesmon/serde,Hoverbear/serde,3Hren/serde