Added an immutable Semiring type with a probability_semiring instantiation
Changed compose to use the semiring of the Wfsts and adjusted Wfst constructor.
Seemingly got composition working using a parameterized probability semiring
added the Tropical semiring
Added a Log semiring. We got more semirings than a pack of burger rings.
Renamed compose to compose_basic
Modified design-questions.txt
Merge branch 'semiring'
Merge branch 'master' of git://github.com/oadams/Fst.jl