add utility/integer_sequence
add utility/is_callable
add utility/unrefcv
Ported v2 repository seek directive to x3 extensions
Flattened extensions directory.
Merge pull request #23 from vtnerd/feature/x3-seek

x3 Extensions Seek Directive
Merge pull request #21 from jamboree/push/utilities

Added several utilities: integer_sequence, is_callable, unrefcv