Decode is missing nil handling / common basic coercions

[mikesimons]

As per the initial comment on #21 I noticed that Decode will not handle quite a few common cases where there may need to be some basic coercions.

Of particular note in reference to #21 is the fact that a nil in any value coming from mruby will fail at one of the unknown type checks.

For nil specifically I think that using zero types would be most appropriate.
But there are additional cases that it would be nice to handle for completeness.

Some examples:

• decodeString doesn't handle floats or boolean
• decodeFloat doesn't handle fixnum
• decodeInt doesn't handle float (not sure if we should handle this given information loss)
• Nothing handles nil

I propose a table of coercions is produced on this issue and an implementation can stem from that.

[mikesimons]

Here is a first (fairly arbitrary) stab aiming to keep it simple.

Horizontal types are ruby
Vertical types are golang

Array / map conversions will follow key / value type conversion rules for each element. (e.g. map[string]float would follow string conversion rules for keys and float conversion rules for values).

Structs, user types & enums are not covered due to potential complexity of mapping.

I'm not sure if float -> int should be allowed due to precision loss. Dynamic languages typically allow this though.

It's also quite strict and does not allow (for instance) Fixnum -> bool conversion.

	TrueClass	FalseClass	NilClass	String	Fixnum	Bignum	Float	Range	Array	Hash	Symbol
Float	invalid	invalid	ZeroType	atof	FloatValue	FloatValue	FloatValue	invalid	invalid	invalid	invalid
String	"true"	"false"	ZeroType	StringValue	StringValue	StringValue	StringValue	invalid	invalid	invalid	StringValue
Array	invalid	invalid	ZeroType	invalid	invalid	invalid	invalid	per-elem	per-elem	invalid	invalid
Map	invalid	invalid	ZeroType	invalid	invalid	invalid	invalid	invalid	invalid	per-elem	invalid
Int	invalid	invalid	ZeroType	atoi	IntValue	IntValue	?	invalid	invalid	per-elem	invalid
Bool	true	false	ZeroType	invalid	invalid	invalid	invalid	invalid	invalid	invalid	invalid

Due to the breadth of work implementing this will require I will await feedback before proceeding.