Add SchemaBuilder

As promised, here is the **`SchemaBuilder`** to create/alter/drop table easily.

The code has all necessary Java doc and unit test coverage is 91% for classes and 96% for line coverage

I also added dependency to **`assertj`** with scope **`test`** so that assertions are more fluent than the classical **`assertTrue`**  and **`assertEquals`** 
It works for me:

    create table foo (s text primary key);
    insert into foo (s) values ('bar');
    alter table foo alter s type text;