Add readavailable for AsyncSockets
Remove wait_readavailable_filter, use wait_readnb
Make special matrices part of matrix factorizations in the manual
Add Clang.jl in the calling C++ section
Merge pull request #2831 from danielmendel/add-readavailable-from-stream

Add readavailable for AsyncSockets
deprecate readdir(::Cmd).
pkg2: use readdir instead of run(`ls`) for massive speedup (duh).
pkg2: for now trying to minimize custom types.
pkg2 wip: bug fixes and some refactoring.
add Base.Graphics module. ref #1609

The two basic pieces are geometry and drawing. Drawing is based on a
minimal subset of the Cairo rendering model. I'm not sure whether this
is appropriate; for example we might want instead a stateless
deferred-mode API, or nothing about drawing in Base at all.