Expose the ttl and install_cache properties on CachingObtainer.

They're "public" anyway, in the sense that they're passed into
__init__(). But this allows the PantsObtainer subclass of CachingObtainer
 to access them in a natural way and use them to initialize the temporary
CachingObtainers it delegates to.

Reviewed at https://rbcommons.com/s/twitter/r/267/

Signed-off-by: John Sirois <jsirois@twitter.com>
Squash exceptions even if user sys.exits

Before, it was possible to short-circuit the exception squashing code in
pex.execute through a sys.exit.  This wraps the execute logic in a
try/finally and increments twitter.common.python to 0.5.2.
Actually print and reraise exceptions in PEX.execute.  Increment to 0.5.3

Reviewed at https://rbcommons.com/s/twitter/r/271/
Increment twitter.pants to 0.0.20.

Reviewed at https://rbcommons.com/s/twitter/r/275/
Do not re-raise SystemExit exceptions

Reviewed at https://rbcommons.com/s/twitter/r/276/
Fix site scrubbing to include site.getsitepackages if available.

Reviewed at https://rbcommons.com/s/twitter/r/289/
Don't follow rel links by default, mirroring pip default behavior.

We should probably make this configurable via PantsObtainer in pantsbuild,
but for now I'm disabling it hard, which mirrors pip 1.6.x series semantics.
This is primarily to fix https://issues.apache.org/jira/browse/AURORA-368.
There's really no good reason to pull rel links.  The only exception we have
in practice is elementtree but I would both 1) love to see that dependency
go away and 2) we can always cache it on raw.github.com if we continue to
rely upon its source as a dependency.

Reviewed at https://rbcommons.com/s/twitter/r/293/