Add account_balance to update customer request.

Also fix all non-passing tests. Most were due to using test
card numbers that no longer appear to work.
Fix MSBuild paths to allow for embedding in other solutions.

When using the project as a library dependency in another solution,
the MSBuild targets should still resolve to the correct
stripe-dotnet locations.
Add Stage configuration.
Merge pull request #14 from ChadBurggraf/master

Add account_balance to update customer request.