Support passing in a string for balance id

[duckmatt]

Fixes #293 with:

• Adds support for full 36 bytes of balance id (4 bytes type followed by 32 bytes for the body)
  • Keeps support of 32 bytes to keep it as a non-breaking change
• Allows for balance id to be passed in as a string

The tests I've added match the expected behaviour of the tests from the Java SDK which cover the same functionality: https://github.com/stellar/java-stellar-sdk/blob/32331b3a5a4cf62eb6820b5e975032b9ab13887c/src/test/java/org/stellar/sdk/OperationTest.java#L757

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.