This makes the plural rule types and functions, like  `uplrules_open`
or `UPluralType` available for use.

Since these changes are as a rule large but mechanical, let's offload
them to this separate commit.

Issue google#127

This should have been done when 0.3.0 was released.  It would be
nice to automate the process completely.

Issue google#127

Plural rules determines what gets printed when one needs to format "1 file"
or "10 files" for human consumption.  Each language may have very own rules
about what number is considered singular, what is considered plural.

Languages have different rules about the numbers subject to the rule, and
even about the number of plurals that are used.

Closes google#127

v0.3.1 will not be released for some time.

See issue google#127

Issue google#127.

Hides the docs by default, since we don't want the
unsafe function to be widely used.

Fixes a typo in the doc formulation.