cleanup
copy factory methods

* Add static copying factory methods to builder. The factory methods
  take either a builder or a value and copies all fields.

* Add a builder() method to values that can be used to copy the value
  into a new builder. The builder() method can be exposed by adding it
  to the value type definition.
readme: bump release version to 0.3
Merge pull request #3 from danielnorberg/copying

Copying