Merge pull request #5 from tmaier/issue-1

Move range parsing to separate classes (Fixes #1)
Order years in range [Fix #7]
Remove reissue parser [Fix #8]
Bump version to 0.0.2
Add BeforeParser
Add AfterParser
Remove obsolete initialisers from AfterParser and BeforeParser
Change after and before parsers to return nil instead of Float::INFINITY

Breaking change: Empty or invalid input returns nil instead of 0 for #year_low and #year_high