Implement SplFileObject::SKIP_EMPTY

Summary: Also fix a bug, where READ_AHEAD would cause an endless loop when iterating over an SplFileObject,
caused by rtrim() on false.

Fixes #4285

Closes https://github.com/facebook/hhvm/pull/4287

Reviewed By: @​svcscm

internal_phabricator

{sync, @​type="child", @​parent="github", @​parentrepo="facebook/hhvm", @​parentprnum="4287", @​parentprfbid="405144262970675"}

Differential Revision: D1691778

Pulled By: @fredemmott
Use MemoryManager to allocate Semaphore resources

Summary: Instead of ~Semaphore calling sweep, move the cleanup code
to ~Semaphore and let the default sweep function invoke it.
(in this case, it's safe to do that).

Reviewed By: @jdelong

Differential Revision: D1719986
Link to "getting started" in readme.md

Summary: New birds-eye overview wiki page we're going to start pointing
people to.

Reviewed By: @JoelMarcey

Differential Revision: D1719984

Signature: t1:1719984:1417729554:8b8194256da18fcaf33bb65238b1a5344ed0e07b