Added support for DESC and NULLS FIRST/LAST to index keys

Fixes issue #3
Function get_index_keys() renamed to get_order_by()

It was a FIXME item in the source. Reasonably so.
Added support for COLLATE to index keys
Added pg_repack runs to check the get_order_by definitions work
Added --moveidx command line option

The option is only parsed, not implemented yet.
Added tests for the namespace change feature

Currently not passing as --moveidx not implemented yet.
Merge remote-tracking branch 'piro/issue-3'

Conflicts:
	bin/expected/repack.out
	bin/sql/repack.sql
Merge branch 'master' into change-namespace
Added check for target tablespace existence
Test fixed after change 66ae2f33a74943d75e6049603fa07eacf46fa1be
Fixed index change namespace test
Added implementation for --moveidx

Note: if original namespace is "foo bar", repack_indexdef gives a bad
result. This is weird as apparently skip_ident can deal with spaces in
a quoted identifier. Committing as I'm going home, will deal with that
later.