minor fixes to `setDT`

    `setDT` lost ExtPtr ref. is now fixed.
    Class attributes to data.frame are now preserved.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1086 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
+ is.sorted moved from setkey down into fastorder.
+ fastorder returns NULL if sorted
+ fastorder now accepts vector as well as list of vector
+ Added vector isorted to fastorder.c and hooked up
+ isSortedList now accepts a subset of columns
+ bench()'s results now reflect sortedness nicely and make sense

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1087 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
+ setDT returns the data.table again
+ added more checks to bench()

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1088 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'refs/heads/r-forge-git-svn-local'

Conflicts:
	pkg/R/data.table.R