Add smallserial and bigserial column types

[liolin]

Add the column types SmallSerial and BigSerial for Postgres.

This should close #111

[liolin]

Both travis builds are failing because of a compiling error of the travis-crate:

error[E0277]: a value of type [_] cannot be built from an iterator over elements of type PathBuf
--> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/cargo-travis-0.0.11/src/lib.rs:307:53
|
307 | &doc.map(|entry| entry.unwrap().path()).collect(),
| ^^^^^^^ value of type [_] cannot be built from std::iter::Iterator<Item=PathBuf>
|
= help: the trait FromIterator<PathBuf> is not implemented for [_]
error: aborting due to previous error

Did I something wrong or is this a known problem of the travis-crate?