Add a factory for the wrapper

that looks as the accumulo.version defined in the pom file.  Added a
system property to exec:exec and test commands.  Only supporting 1.5.0
currently, but installed 1.5.1-SNAPSHOT locally and it seems ready to
go.  Would just need to add a MiniAccumuluClusterWrapper1_5_1.java and
handle that version in the MiniAccumuloFactory.