Allow GraphQLFloat to be represented by float64

[sogko]

This PR addresses #130 and depends on PR #123

Relevant commit changeset:
c7fc493 Updated graphql.Float coercion

• if value type is explicitly float32, return float32
• if value type is explicitly float64, return float64
• if value type is int, return float64
• if value type is string, use strconv.ParseFloat() to return float64
• for everything else, use system-default

[coveralls]

Coverage increased (+0.003%) to 90.754% when pulling c7fc493 on sogko:sogko/float64 into a5cf5f2 on graphql-go:master.

[coveralls]

Coverage remained the same at 90.754% when pulling 3582a63 on sogko:sogko/float64 into 4a8d417 on graphql-go:master.