accurate sind, cosd functions
`using Nonexistent`: better behavior loading unfound names.

    julia> using FooBar
    ERROR: FooBar not found
     in require at loading.jl:45

The core change is that `find_in_path(name)` now returns `nothing`
when `name` isn't found and various functions that call it must
check explicitly for `nothing` as a return value and throw an error
as appropriate. In particular `require` and `reload` now raise an
error instead of just forging ahead trying to load a file we know
doesn't exist, which resulted in less-than-clear error messages.
fix #4080, don't try to deepcopy Tasks
Revert "Fix beta function for BigFloats"

This reverts commit 5b3745294c8cfd4bb3c33ff2482ae256f5e138b7.
add lgamma_r for BigFloat, make beta() work
doc: patch up the development docs for the Pkg rename [fix #4081].

Obviously better docs that don't depend on Pkg1 are in order.
Merge branch 'master' of github.com:JuliaLang/julia
Merge pull request #4079 from simonbyrne/trig

accurate sind, cosd functions
small formatting changes to sind/cosd improvements
Merge branch 'master' of github.com:JuliaLang/julia into jb/repl_show_to_display
update with stevengj's comments