remove versioning code.
remove versioning option from post model.
remove versioning table.
specify upper bound on rails version since this version is not compatible with > AR4.
add rdoc development dependency.
add post install message about versioning support.

Conflicts:
	globalize.gemspec
bump version.
Update changelog.
Merge pull request #310 from globalize/gut_versioning_3-0

Extract versioning code