Move range parsing to separate classes

All parsers are derived from RangeParser class

Based on Olsen, Russ: Eloquent Ruby (2011) pp. 250-253
Move initializer to RangeParser

All parsers inherit from RangeParser. It is not necessary to have this method multiple times.
Merge pull request #5 from tmaier/issue-1

Move range parsing to separate classes (Fixes #1)