Clarify short-term limitations

Inspired by #5, #6, and various questions posted on our Hacker News thread.
Update README.md

Small tweak.
crash due to use_citusdb_select_logic on postgres is fixed (#10)
Update error message

When the "use_citusdb_select_logic" configuration parameter is set
to "true" on postgres, an error report is produced. This commit is
done for updating the error report.
White space change

According to CitusDB coding conventions, it is required to have a space
after "if".
Merge pull request #29 from citusdata/feature/citus_integration#10

Crash due to use_citusdb_select_logic on postgres is fixed

cr: @jasonmp85
Move test code to subdir. & build hierarchically

See [Recursive Make Considered Harmful][1] for a discussion of merits.

[1]: http://aegis.sourceforge.net/auug97.pdf
Split existing tests into two files

To make management better in the future.
Add a ton of distribution metadata tests
Remove me before PR
IWYU cleanup
Add tests for DDL generation
Add tests for DDL extension
Add cleanup steps to various tests

To avoid leaving cruft between tests.
Add AppendShardIdToName tests
Cleanup
Shard creation testing

Phew.
Add shard pruning tests
Add repair_shard tests; fix bug

Next up: integration testing.
Use templating for repair shard tests

Needed because of the port number.