Fix #6372

This adds the package to REQUIRE on Pkg.clone() if it exists in
METADATA. This means that calling Pkg.clone() on a repo that's on an
old tag will result in the package getting updated to the latest
release, but that seemed preferable to making Pkg.clone() on a package
that's on a tag result in a pinned package.
Implement grisu algorithm in native Julia.
Merge branch 'master' of https://github.com/JuliaLang/julia into jq/grisu
Export the grisu method to allow importing by Printf module. Should fix travis failure.
Merge branch 'master' of https://github.com/JuliaLang/julia into jq/grisu
realmax(Float16) now prints '65500.0' instead of '65504.0' in accordance with the grisu alogrithm (printing the shortest form that can be read back in accurately).
1st attempt at removing all double-conversion library traces.
Add double-conversion LICENSE to grisu folder. Tweak .gitignore
Merge branch 'master' of https://github.com/JuliaLang/julia into jq/grisu
Merge branch 'master' of https://github.com/JuliaLang/julia into jq/grisu

Conflicts:
	deps/Makefile
Merge branch 'master' of https://github.com/JuliaLang/julia into jq/grisu
Evaluate and print parts of comparisons in `@test`.
Adds a resultexpr field to Failure and Success.
Add Dates module to Base
Added for spatted printf tests
Revamp netload tests to more verbose and resiliient style
Fix nettest to run on multiple workers again
Whoops, accidentally reduced test case size
Update Package Development documentation
more explanation of abstract types

Several points concerning abstract types eluded me when I first read the manual; I have written additional paragraphs to elucidate these points.
more explanation of immutable

Some aspects of 'immutable' escaped me when I first read the manual and were clarified by Stefan Karpinski.  II think other new users might benefit from this clarification, so I put some additional paragraphs in the manual.