Add ability to specify columns for multiget queryColumns

ColumnFamilyTemplate has a queryColumns method for a single key where you can specify which columns to return.

public ColumnFamilyResult<K, N> queryColumns(K key, List<N> columns)

There was no multiget equivalent though i.e:

public ColumnFamilyResult<K, N> queryColumns(Iterable<K> keys, List<N> columns)

Only a method that requires you use a ColumnFamilyRowMapper:

public <V> MappedColumnFamilyResult<K,N,V> queryColumns(Iterable<K> keys, List<N> columns, ColumnFamilyRowMapper<K, N, V> mapper)

https://groups.google.com/forum/?fromgroups#!topic/hector-users/xsS9N8ld6L4