Compatibility with Julia 0.7

[omus]

I was spent some time getting more familiar with the NamedTuples code and I think the best way to have this package be both Julia 0.6 and 0.7 compatible would be to:

1. Remove some of the NamedTuples.jl features that aren't supported in 0.7 (e.g. getindex(::NamedTuple, ::AbstractVector) -> NamedTuple)
2. Make abstract type NamedTuple end in 0.6 be abstract type NamedTuple{K, V<:Tuple} end to be more consistent with 0.7. With this change we should also be able to deprecate the @NT(a::Int64, b::Float64) and @NT( a, b )( 1, "hello") syntax.