MAJOR Switched to standart scheduler.

Now we will use standart Python scheduler instead of manual work with os.sleep.
MAJOR Remove '--once' option.

Now the time period equial to zero will have the same as '--once' option. Also '-w' is not a mandatory anymore and it's default value is zero.
DOC Updated description.

Changes:
 -- removed description of '--once' option;
 -- removed 'Hints' section;
 -- extended description of '-p, --period' option.