v0.7.5

• Keypair.constructor now requires type field to define public-key signature system used in this instance (so Keypair can support other systems in a future). It also checks if public key and secret key match if both are passed (to prevent nasty bugs).
• Keypair.fromRawSeed has been renamed to Keypair.fromRawEd25519Seed to make it clear that the seed must be Ed25519 seed.
• It's now possible to instantiate Memo class so it's easier to check it's type and value (without dealing with low level xdr.Memo objects).
• Changed Asset.toXdrObject to Asset.toXDRObject and Operation.operationToObject to Operation.toXDRObject for consistency.
• Time bounds support for numeric input values.
• Added browser prop to package.json.