Add by_semester method, for the 6-month intervals of the year

[morellan]

I'm using this extra method to check if something happened in the first half or second half of the year, I hope this could be useful for others.

[johnnyshields]

Hi, this looks fine to merge, however, can you first help fix the failing Travis tests? (I realize they aren't caused by your PR)

[morellan]

Happy to help, but I'm not sure where to start since most of the errors are caused by some problem in the CI

$ mysql -e 'create database by_star_test;'
ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)
The command "mysql -e 'create database by_star_test;'" failed and exited with 1 during .

[johnnyshields]

Thanks! Will merge when CI is green.