New algorithm to determine the duration of the super magic token

* flo_check_homework/main.py: when the super magic word entered by the
user contains a decimal point, the duration of the super magic token is
determined using a new method. This should make it practically
impossible for unauthorized personnel to tamper with the token duration
as the super magic word is being entered.

* flo_check_homework/main.py: use the decimal module to ensure that the
digits seen by the program are the same as those entered by the user.
Documentation update

* INSTALL.txt: better explain the pip-based installation method. In
particular, indicate in which situations it is safe or unsafe to use.

* README.i18n: document that PyQt version 4.10.4 or later should be
sufficient to regenerate the .ts files.
Remove unused lines from MANIFEST.in that cause warnings

* MANIFEST.in: remove "prune flo_check_homework/images/unused" and
"global-exclude .gitattributes" because they cause warnings upon
installation that may frighten users and only provide a safety net
against hypothetical bugs that don't exist in the current version.
Release 0.11.0