Implement the unsigned integer type

[zorchenhimer]

This implements encoding and decoding unsigned integers.

Also included are test cases for both encoding and decoding.

[ianlopshire]

@zorchenhimer Thanks for taking the time to submit this PR! Everything looks great!

The only thing I see missing is listing the uint types as encodable here:

go-fixedwidth/encode.go

Lines 19 to 24 in 54451b9

	// In order for a type to be encodable, it must implement
	// the encoding.TextMarshaler interface or be based on one
	// of the following builtin types: string, int, int64,
	// int32, int16, int8, float64, float32, or struct.
	// Pointers to encodable types and interfaces containing
	// encodable types are also encodable.

As soon as that is updated I will merge and cut a release! Thanks again for the PR!

[zorchenhimer]

I knew I was missing something, lol. I've added the types in there.

[ianlopshire]

@zorchenhimer Thanks again for this PR! I'll cut a release as soon as #41 lands on master!