Add a link to the latest finished eval of a jobset
Support passing a jobset evaluation as an input

All successful, non-garbage-collected builds in the evaluation are
passed in a attribute set.  So if you declare a Hydra input named
‘foo’ of type ‘eval’, you get a set with members ‘foo.<jobname>’.  For
instance, if you passed a Nixpkgs eval as an input named ‘nixpkgs’,
then you could get the Firefox build for x86_64-linux as
‘nixpkgs.firefox.x86_64-linux’.

Inputs of type ‘eval’ can be specified in three ways:

* As the number of the evaluation.

* As a jobset identifier (‘<project>:<jobset>’), which will yield the
  latest finished evaluation of that jobset.  Note that there is no
  guarantee that any job in that evaluation has succeeded, so it might
  not be very useful.

* As a job identifier (‘<project>:<jobset>:<job>’), which will yield
  the latest finished evaluation of that jobset in which <job>
  succeeded.  In conjunction with aggregate jobs, this allows you to
  make sure that the evaluation contains the desired builds.
Allow passing a specific build as an input

Fixes #62.