Allow internalClient to accept context.Context and account IDs as int

[ppacher]

As a first step towards a new client that uses int for account IDs and supports context.Context (see #12) this PR updates the internal client to deal with int and context.Context. It also adds a new Client2 and updates the existing Client to use the new version in the background. Since this PR does not change any public APIs it should count as backwards compatible.

@cainlevy let me know if I should change something