Bugfix for CAF interpreter

The Caf interpreter test was not actually testing the new
Number=R caf field.  Updated the tests to actually test this,which
broke.  Fixed how we were calling info_for_allele to return the
expected value.
Use constant TAG
Don't test multiple versions of CAF

The multiple versions of CAF are exercised in the DbsnpAfParser,
so we don't need to combinatorially test all of them in this
class which uses the parser.  We can just test the new one.
Merge pull request #31 from apregier/caf_bugfix

Caf bugfix