Statically generate resource URL's from the marketplace_uri

[ajsharp]

• 41 - Memoizes the marketplace_uri in Balanced::Marketplace.marketplace_uri

• 45 - Addresses some logical bugs generating predicate methods

• 44 - Fixes a bug with the Balanced::Error class

This PR is primarily a performance enhancement. The change introduced in f3df3ae changes resource URL generation from using properties returned by the Balanced::Marketplace.my_marketplace method to statically building URL's based upon the marketplace URI, which is now cached. So, instead of relying on the GET /v1/marketplaces/:id call to give a url like /v1/marketplaces/:marketplace_id/accounts, we simply construct this string in the code if we have the marketplace_uri available.

We discussed memoizing the entire .my_marketplace method, but there are dynamic properties returned in that response that make that solution infeasible.

[ajsharp]

@mahmoudimus @matin @mjallday Would love to get your thoughts on this stuff.

[mahmoudimus]

Thanks for this Alex. I've reviewed it and I think it's good to go.