Add currency and price to Content validation

[fromheten]

Fixes #25

Currently the Content record type does not make sure we have currency and amount specified.

• [x] We need to specify currency as any we can know the exchange rate for. Currently, we have not implemented such thing, so the only one is satoshi. So currently, it can only be satoshi. Easy to validate ;).
• [x] We also need to specify amount, which is an integer (I think). So, if we support a currency such as Euro the base amount would have to be something like a milli-euro (1/1000 euro). This one can get hairy. Maybe better to have it as a float? It's not like we are going to do a lot of calculation about it, and people can always enter the amounts in a way that feels natural to them. Idk, let's think about this one @mattgstevens.

[mattgstevens]

We will want to store and do maths as integers since floating point math is not precise http://stackoverflow.com/questions/588004/is-floating-point-math-broken

When we support other currencies, we can translate from a "natural" currency number (like $5.67 or 5,67€) to our internal representation in pennies 567. Stripe is also storing the amount as an int for similar reasons https://stripe.com/docs/api#charge_object-amount

[mattgstevens]

Looks good to me, opened issue #28 with the above comment for when we add currencies other than satoshi

[fromheten]

Re: Integers for currencies: I'm with ya man, that is the way to go. I guess we just have to decide how precise we can be - or just use the lowest that is common in the region where the currency is used (so x.xx for dollars and euros and pound, x for yen, etc).

[fromheten]

What the hell, Stripe write "öre" in here for describing pennies?! Is that a word in English?