+ Fixed bug in svn validate_version with undefined reference to
self.version
+ Add get_tag_name method.  This allows users of Rez (in place of a more
  advanced configuration system) to more easily support different tag
naming
  conventions without having to apply larger patches to the SCM plugins.
+ Made the tag name a property as this creates a slightly nicer api.
+ Add get_version_from_tag method to return a Version object from a tag
string.
  This supports tags which are not just a version number, and potential
  support for changing the tag name strategy over time.
+ Update SCM plugins to use tag_name property instead of package
version.
+ Add 'replace' command which is a light wrapper on setenv.
+ Fix a bug in Rexnamespace so that the vars attribute is now updated
whenever
  a new command recorder is set (as opposed to just on __init__).  This
is
  required to ensure commands are recorded correctly.
+ Update the recording of commands to support 'replace'.  This allows us to
  replace a variable set by another package.  This can be restricted by setting
  a list of friends on the replace command, which only allows it to replace a
  value if it was set in one of those packages.
+ Tweak error messages.
+ Tweak error messages.
+ Polishing changes.