I'm not a lawyer and the advice listed below is just my common sense, I would advise you to consult with a lawyer regarding your question.

In your case I would just license it - you can either put it under any of the open source license out there or use a commercial license generator - http://www.binpress.com/license/generator (I'm one of the founder at Binpress).

By the way you mention having an employment that is relatively lenient which is good.

As an employer I would still advise you to either include an appendix or send an email to your legal department listing your side projects, open source projects you contribute to and/or anything else you think should be included there. It is probably being over cautious on your side but remember than employment is like a marriage; everything is fine and nice when they get you to sign on that contract but you never know what will happen when you leave your job to seek your next opportunity.

Additionally if you are in the US, you probably signed on some sort of PIIA (ROPRIETARY INFORMATION AND INVENTIONS AGREEMENT) which will assign anything that you've developed or invented back to the company, some will make it lax and some will be stricter, I would guess most employers will be stricter in order to avoid any potential IP problem in the future.

However if you are in California, know your rights. There's the California Labor Code Section 2870 which states the following:

  (a) Any provision in an employment agreement which provides that an
  employee shall assign, or offer to assign, any of his or her rights 
  in an invention to his or her employer shall not apply to an invention 
  that the employee developed entirely on his
  or her own time without using the employer’s equipment, supplies, facilities, 
  or trade secret information except for those inventions that either:
  (1) Relate at the time of conception or reduction to practice of the
  invention to the employer’s business, or actual or demonstrably anticipated 
  research or development of the employer; or
  (2) Result from any work performed by the employee for his
  employer.
  (b) To the extent a provision in an employment agreement purports to require
  an employee to assign an invention otherwise excluded from being required to be
  assigned under subdivision