Log Message:
Changes 0.04:
* To detect a perl with ${^GLOBAL_PHASE}, check for the feature itself instead
  of a specific perl version.
* Update the documentation to reflect the use of ${^GLOBAL_PHASE} if available.
* Stop depending on Scope::Guard for the tests.
* Upgrade ppport.h from version 3.13 to 3.19.

Changes 0.03:
* Drop the XS code on perl versions recent enough to have ${^GLOBAL_PHASE}.
* Drop code to support perls older than 5.6.  We've always been depending on
  5.6 anyway.
  + Use XSLoader without a fallback to DynaLoader.
  + Use our instead of use vars.