Added composer with gitignore.
I picked the wrong phpunit package.
Added bin directory.
Added PSR-4 config.
Added unit tests. Test string taken from https://code.google.com/p/bitcoinj/source/browse/core/src/test/java/com/google/bitcoin/core/Base58Test.java
Added string tests.
Adjusted the structure of this test code.
Improved code so that encoding now works full with all tests passing.