Added 2 missing column types and a section on dropping timestamps and SoftDeletes

I noticed that `mediumInteger` was missing. I added it to the list taking in account the alphabetical order.

edit:
I added 2 more commits:
- Added `nullableTimestamps` column type
- Added a section about dropping `timestamps`, `nullableTimestamps` and `SoftDeletes` using `dropTimestamps()` and `dropSoftDeletes()`.