Improved email message format.

- Temperature rounded to 1 decimal.
- Added 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.
- All configuration tests and parameters retrieval encapsulated
  in respective library function.
Fixed merging global config with local one.

- 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.