Fix #606

If redshift.conf is not found, fall back to formerly used path.

Travis: Run "brew upgrade python" to install Python 3 on macOS

As of March 2018, Travis CI fails on macOS:

Error: python 2.7.14 is already installed
To upgrade to 3.6.4_4, run `brew upgrade python`

This commit fixes the issue.

Fix #335: Set period to none when disabled

This allows for hooks to be run when redshift is disabled

options: Split up parse functions

Update translation files

Improve man page

Preserve command-line option

Changes each adjustment method to take a preserve parameter when
setting the temperature instead of parsing the preserve option
from the command line/configuration file. This helps resolve the
issues around #513:

- This allows the preserve option to be implemented as a
  command-line switch (-P). This switch _disables_ the preservation
  of existing gamma ramps. Having a command-line switch makes it
  easier to use directly with manual or one-shot mode.
- The preserve options is on by default, so continual mode as well
  as other modes will default to applying the color adjustment
  on top of the current gamma ramps.
- Preserve is always disabled in reset mode so resetting works
  as expected again.

Cleanup redshift.c   modules   options

Create options.c which contains functions for initializing and
parsing options from the command line and the configuration file.
Program options that were previously local variables in main() are
moved to the options_t struct.

Add help message to redshift-gtk

Updated Swedish translation

quartz: Fix incorrect use of display identifier

The quartz (macOS) adjustment method incorrectly used the display
index instead of the associated CGDirectDisplayID. Incidentally this
happened to work too but was incorrect.

Fix bug where error code was not handled

Fix missing checks and verbose output

Rearrange checks and verbose output in main() to bring back checks
that were accidentally left out when time-based adjustments were
enabled. This includes the output indicating the daytime/night
temperatures and the check that the temperatures are valid.

Add documentation for time-based adjustment

Travis: Enable AppArmor install