Add lbr as an aliase for pounds

[guillec]

In the industry I work in, lbr is commonly used for pounds and the UNECE.org recognizes it as an abbreviation for pounds.

http://www.unece.org/fileadmin/DAM/cefact/recommendations/add3lmdt.htm

[guillec]

@olbrich please let me know if this is something you would be interested in merging. I am a little confused on the test failure so if you could point me in the right direction I can fix that up.

[olbrich]

I'll consider adding it. In the mean time, it is possible for you to add your own abbreviations... https://github.com/olbrich/ruby-units#redefine-existing-unit.

The test failure is related to the upcoming removal of mathn from the ruby stdlib. It's not your code.

[guillec]

Thank you.