Added revision view column.

Added a column that can be added to the job views and which shows the hg
revision the job checks out. For non-hg jobs the column simply shows
nothing.
Fixed possible problem with expecting a Job as parameter.

Changed expected object to be any Item. Also removed Guava dependency
and clarified column name.
Renamed column header.
Merge pull request #49 from andreas-schilling/master

Added revision view column