Merge java-stellar-base into java-stellar-sdk

[bartekn]

We decided to merge java-stellar-base and java-stellar-sdk into one repository and deprecate the former. The way Java works makes it difficult to export other package classes from the “parent” package, like we do in JS to separate low-level objects from classes responsible for network connections.

What is going to change

• All classes representing responses from Horizon server (and their deserializers and GsonSingleton which aren't part of public API) are moved to org.stellar.sdk.responses package.
• All org.stellar.base.xdr classes are moved to org.stellar.sdk.xdr package.
• All org.stellar.base classes are now in org.stellar.sdk package.
• TransactionBuilderAccount interface stays so it's easy to build a transaction using a raw KeyPair and sequence number in org.stellar.sdk.Account object but it's also possible to use response from Horizon represented by org.stellar.sdk.responses.AccountResponse object.
• Deprecated methods will be removed:
  • KeyPair.fromAddress (use KeyPair.fromAccountId)
  • KeyPair.getAddress (use KeyPair.getAccountId)

This PR diff is quite hard to read so I created a javadoc for the changed API. It can be found here: http://bartekn.github.io/java-stellar-sdk-proposal-doc/

Breaking changes

This changes will be released as version 0.1.0. We will support 0.0.* versions of sdk and base for unspecified amount of time (we will disclose it after merging this PR) while encouraging integrators to switch to version 0.1.0.

[myrle-krantz]

Looks good to me.