fix building transaction with muxed account source

[fracek]

This PR also includes 9891652693feeb1013c8e63c67f0ad785708ae03, 7321d7abf8818761b9b28038f945e862f900cfb2 and 97ba78e2e02a0fcc94862d3c4746d7c655a3da63.

Similarly to the js sdk, I added a TransactionBase class for Transaction and FeeBumpTransaction. I renamed the Transaction.Builder to TransactionBuilder but kept the inner class with a deprecation warning for backward compatibility. Account did not play well with the new MuxedAccount, so I added an overload of the ctor to accept MuxedAccount. If the user passes a string to the ctor, we assume it's a normal keypair and parse it with KeyPair.FromAccountId. That way we are backward compatible with old code.

Types of changes

• [ ] 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.
• [x] 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.

[coveralls]

Coverage increased (+0.2%) to 84.332% when pulling cb439444263c4c41aa408876b50d3e705d325ed6 on fee-bump-tx into 9891652693feeb1013c8e63c67f0ad785708ae03 on protocol-13.