Software as top level entity

According to the discussion on

https://groups.google.com/a/pyfarm.net/forum/?hl=de#!topic/discuss/UnUPJPcfRVg

this consolidates the JobSoftware, AgentSoftware, JobTag and AgentTag tables into two tables with shared tags and software items for both jobs and agents (and possibly others in the future, like jobtypes) and creates intermediate association tables to allow for many to many relations between Agent and Tag or Software as well as Job and Tag or Software, respectively.