Many amendments to PHP code.

Reading temperature
===============
- Current and limit temperature are read from system files instead of from vcgencmd utility.
- The utility vcgencmd is not preinstalled in all distributions, especially not
  in minimalistic ones. It is recommended to read temperatures from system files:
  - Current temp: /sys/class/thermal/thermal_zone0/temp
  - Limit temp: /sys/class/thermal/thermal_zone0/trip_point_0_temp
- The reading of the limit temperature has been included for future use. Is is
  the maximal allowed temperature for Raspberry Pi. Usually, at reaching this
  value the system is forced to shutdown.
- The naming of temperature variables has been unified.

Getting IP addresses
===============
- IP address lists all active and broadcasting interfaces.
- The Raspberry Pi can be connected to LAN concurently with
  more interfaces, e.g., with ethernet (eth0) and WiFi (wlan0).
- The function getServerAddress returns sorted list of all IPs,
  so that it needs no input argument anymore.
- If there is no active broadcasting interface available,
  the IP address reads "n.a." clearly indicating this fact.
- For getting IP address list the utility hostname is used
  (hostname -I) instead of ifconfig and regex-ing.

Formatting email messages
====================
- Temperature rounded to 1 decimal although it is read and logged
  with 3 ones (in mili centigrades).
- Appended degree sign to the temperature item.
- If the displaying hostname is enabled in the configuration file,
  the hostname is appended to the subject of a message for
  better filtering in email clients.
- All configuration tests and parameters retrieval encapsulated
  in respective library function instead of monitor.php.

Merging global and local configuration
===========================
- Fixed merging in-line configuration arrays (notification).
- Originally employed PHP built-in array_merge does not merge
  in-line arrays, i.e., entire notification section.
- PHP built-in function array_merge_recursive does not merge
  items with integer keys and leaves duplicated items. So
  it is useless for the configuration merging.
- Custom function array_merge_deep has been created to merge
  arrays deeply without items duplication as expected for the
  configuration description in the guide.