fread: use download.file(..., quiet=!show.Progress)

 - fread prints stuff even with showProgress=FALSE when loading URL

 - call download.file with quiet = !showProgress so that the progess
   information can be suppressed.
README: "How to install" link was wrong

 - wiki/Installation doesn't exist
 - correct link seems to be wiki/Download
Merge pull request #863 from kbroman/fix_link

README: "How to install" link was wrong
Revert "README: "How to install" link was wrong"
Since the README on CRAN links to Installation, have changed the wiki page itself back to Installation.

This reverts commit 5a703660bd3545d680db8b8ec937e87bf40c0ea2.
Guess that snowleapard failed because xts was building in parallel at the time (by chance) and hence not available. Changed last() to work if xts is not loaded (its stop() removed).
:= suppressed in a more reliable way not depending on trigger depth. Both if(TRUE)DT[,:=] and `:=` in knitr should now be suppressed. See README for one downside, though. Closes #869.
Added ) to README
.EACHI=NULL exported. Comments for posterity added about := auto-print suppression.
README only
DT[colA==max(colA)] works again without needing options(datatable.auto.index=FALSE). Closes #858.
DT[colA %in% c(... some dups ...)] now ignores the dups again with auto indexing on.
DT[class=='a'] works again with auto indexing (setAttrib didn't like the index name being "class", now prefixed with __)
Don't optimise if .I, e.g. c(lapply(.SD, sum), .I)
Printing test number in last 13 lines of CRAN output resolved.
Don't optimise c(…) in `j` if not wrapped by list().
In j-expr with c(..), .I if unnamed gets named "I".
Merge branch 'Fix-861'

Conflicts:
	inst/tests/tests.Rraw
Update NEWS
Merge branch 'Fix-861'
Merge branch 'master' into Feature-735