Invalid outer join for postgresql

[abondar]

Current join enum have only basic "OUTER" join, which is not supported by postgresql.
how parameter for joins should accept list of JoinType's, so join could be concatenated, or JoinType enum should be extended to have `LEFT OUTER' 'RIGHT OUTER' 'FULL OUTER'.

I can implement it, but would like to hear your opinion on this matter, may be I am missing something, how joins should be built with pypika

[twheys]

Thanks for reporting. This is a really small fix so I just raised a PR. To keep it simple I just added enum values for these three, since not every combination is possible, and since the list of join types is quite small and manageable.