Fix FeeBumpTransaction submit.

[Kirbyrawr]

• Added FeeBumpTransaction in SubmitTransactionOptions.
• Now SubmitTransaction checks if it's FeeBump or not.
• Added FromEnvelopeXdr in FeeBumpTransaction.

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.
• [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 (+0.03%) to 88.812% when pulling 91e4106216d9dd8d4db41618e14236abaa5034e7 on fix-fee-bump-submit-transaction into 0a91922a67ffb07bebfe4b3e3fde9437ca1dda57 on master.