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.
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.
added a paragraph about elementwise

As far as I can see, elementwise operators are never introduced or defined in the manual, so I added a paragraph to introduce and define them.
PriorityQueue: use O(logN) algorithm for dequeue!(pq, key)
Still return a tuple from `peek(pq)`
Fix type-stability of setindex!(::PriorityQueue, val, key) (return val)
a couple small flisp tweaks; use set! less, remove unused stuff
Merge branch 'master' of github.com:JuliaLang/julia
Expand style guide on naming

An attempt to capture the discussion at https://groups.google.com/d/msg/julia-dev/8ty9cQe_uGM/mkk3PiDG17wJ,
particularly https://groups.google.com/d/msg/julia-dev/8ty9cQe_uGM/Qgu9I6lN-LgJ
Merge branch 'master' of github.com:JuliaLang/julia
add 1607 new LaTeX tab completions from the unicode-math package (closes #7657)
docs: use Integer instead of Int in type constraints
Merge pull request #8052 from rfourquet/docfix-integer

docs: use Integer instead of Int in type constraints
Merge pull request #7873 from StephenVavasis/patch-3

more explanation of immutable
Merge pull request #6814 from JuliaLang/sjk/6372

Fix #6372
Merge pull request #8011 from JuliaLang/teh/pq

PriorityQueue: use O(logN) algorithm for dequeue!(pq, key)
Merge pull request #7876 from StephenVavasis/patch-6

added a paragraph about elementwise
Merge pull request #8031 from JuliaLang/teh/styleguide

Expand style guide on naming
Document the breaking changes to PriorityQueue