Any object that inherits from BaseDateTime (including DateTime) has the method
public long getMillis()
According to the API it:
Gets the milliseconds of the datetime instant from the Java epoch of 1970-01-01T00:00:00Z.
So a working example to get the seconds would simply be:
new DateTime().getMillis() / 1000
For completeness, the definition of the Unix Timestamp according to Wikipedia:
Unix time, or POSIX time, is a system for describing instants in time, defined as the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970, not counting leap seconds.
You can also improve it further by removing the magic number division using the TimeUnit API:
import java.util.concurrent.TimeUnit;
TimeUnit.MILLISECONDS.toSeconds(new DateTime().getMillis());