Update column

Added an updateColumn method for Schema.

eg:
```
Schema::table('users', function(Blueprint $table)
{
    $table->updateColumn('username', 'string')->length(240);
    $table->updateColumn('username', 'enum')->allowed(['y', 'n'])->default('n');
});    
```

Due to lack of knowledge:
* Only written the mySql grammar
* Have not written any tests