add support for testing w/ multi-backend

'multi' is now a valid option for 'backend' test metadata. Additionally,
the 'multi_config' test metadata field controls which multi-backend config
is used. 'multi_config' can be set through the command line w/ the "-m" flag.
The default value is 'default'.

There are two preset multi-configs. 'default' contains eleveldb, memory and
bitcask backends. 'indexmix' contains memory and leveldb backends -- backends
which support 2i.
Added parsing of multi_config from giddyup tags
Merge branch 'master' into eas-multi-backend-merged

Conflicts:
	src/riak_test.erl
	src/riak_test_runner.erl
	src/rt.erl
Merge pull request #259 from basho/jrw-multi-backend

add support for testing w/ multi-backend