Parameters for select

Add possibility do like this:
```
Column("IF(d IN ("+Placeholders(3)+"), 1, 0) as stat_column", 1, 2, 3).
Column("IF(e IN ("+Placeholders(len(interfaceSlice))+"), 1, 0) as stat_column2", interfaceSlice...).
```		

1) new method `Column(column interface{}, args ...interface{})` - for add columns with parameters
2) new struct `selectPart` - analog of `wherePart`
3) `selectPart` and `wherePart` now based on `part` and implement interface `sqlSerializable`
4) tests
5) as i see, i didn't break backward compatibility :-)

I saw struct `expr` but didn't touch it. 
If you want change or improve something please give me feedback