Add "requires" parameter validation about raml-spec

[dmartinezg]

The initial drafts had these, requires should be a list.

BUT, If we add it, then we need to add conflicts to indicate that this parameter cannot be used with others.

And then, you will want to say, required only if parameterA == 'valuea' and not if parameterB == 'valueB'.

We call this feature, by the general theme: dependencies and we do not have a clear answer for it yet.

[Philzen]

+1 reg. adding a conflicts property , see #38

At least i'm looking for a way to avoid having statements like "don't use with parameter xy..." in a description. For the example of Oauth2, maybe a smart conflicts property could capture conflicts when a certain named parameter is set to a specific value, i.e. conflicts: { namedParameter: "conflictsIfNamedParameterHasThisValue"}

[Philzen]

@blakeembrey Regarding the lat/lon example - there's a simple alternative to it, namely requiring a combined parameter. I've see that in the google elevation api (?locations=64.330000,7.500000), and in the api i'm currently building we simply refer to the geouri formatting rules defined in RFC5870 - which is effectively the same plus optional altitude and uncertainty values.