Fix bug in reindex.

Some NA values were being dropped, which meant the values was undefined
at the tail end.
Add some docs to column.
Add apply method to Column object.
Fix bug in resetIndices for unordered index.
Bump version to 0.2.3.
Merge branch 'master' into topic/column-sanity
Add orElse method to Column.
Column macros compatible with 2.10+paradise.