Database migration improvements

Added new functionality to Schema::table;

- The ability to rename/change the type of columns, fully supported by MySQL and Postgres, partially supported by SQL Server.

    ````PHP
    // Rename oldcolumn to newcolumn and set the type to blob.
    $table->blob('newname', 'oldname');
    ````

- Added the ability to add enum columns, fully supported by MySQL and Postgres, converted to a varchar on SQLite and SQL Server.

    ````PHP
    // Add new enum column named foo, with bar and baz as possible values.
    $table->enum('foo', array('bar', 'baz'));
    ````

- Added the ability to specify after which column a new column should be added. Supported by MySQL only.

    ````PHP
    // Add new column after existing column.
    $table->bool('new_column')->after('existing_column');
    ````