Enum type for operators instead of unchecked string, added precedence
values and string to enum mapping. Implemented tokenizing of input
string for use in conversion algorithm.

Minor clean ups. Main class still temporarily only for testing purpose.
Will implement command line arguments and set constants next.

Change-Id: I28b49ff9a8e38167cab9d788cd45086bb1779db4
Signed-off-by: qaovxtazypdl <qaovxtazypdl@hotmail.com>