Add ability to run workers at a different nice level from parent.

Closes #15
Add basic ability to reconfigure Einhorn on the fly
Configure Travis
Allow a user-specified renice command
Drop 1.9.x for now
Only log if actually renicing
Don't require root when renice is specified
Improve logging of renicing
Attempt to handle re-exec failures gracefully on reload.

If re-execing ourself fails for some reason, soldier on, instead of
dieing and leaving detached children floating around.n
If a reload fails, clear Einhorn::State.reloading_for_preload_upgrade.

If a reload failed, this was causing further SIGALRMs to not bother
reloading, since they thought we were already doing so. In conjunction
with preloading, this meant we would never upgrade our workers.
Explicitly select :workers when looking at children

Fixes #18
Make the tests run on Ruby 1.9
Switch from EINHORN_FDS to EINHORN_FD_N/EINHORN_FD_COUNT

The space-separated EINHORN_FDS interface was an attempt to pretend to
be compatible with Upstart. Upstart internally supports passing
multiple FDs, but exposes no interfaces that would allow
it. Meanwhile, having the space-separated list of FDs makes some other
applications more difficult.
Merge pull request #19 from ebroder/einhorn-fd-n

Switch from EINHORN_FDS to EINHORN_FD_N/EINHORN_FD_COUNT
Run tests on 1.9.3 and 1.9.2 (since Heroku uses that)
Fix the tests on 1.9.2
Bump version
Fix the docs to match actual behavior of EINHORN_FD_N vars

They actually count from 0, not 1. I hadn't originally intended for
this to be the behavior, but now that the code is written, it seems
preferable to attempting to do a bunch of conversions between
0-indexing and 1-indexing, so I think I'd prefer to keep it.
Merge pull request #20 from ebroder/fix-envvar-docs

Fix the docs to match actual behavior of EINHORN_FD_N vars
Bump version