add support for ordinal days

[alexduryee]

Ordinal numbers (1st, second, 3d, etc.) confuse the library and lead to bad parsing:

1.9.3-p551 :002 > Timetwister.parse('Aug. 3rd, 1886')
 => [{:original_string=>"Aug. 3rd", :index_dates=>[], :date_start=>nil, :date_end=>nil, :date_start_full=>nil, :date_end_full=>nil, :inclusive_range=>nil, :certainty=>nil}, {:original_string=>"1886", :index_dates=>[1886], :date_start=>"1886", :date_end=>"1886", :date_start_full=>"1886-01-01", :date_end_full=>"1886-12-31", :inclusive_range=>nil, :certainty=>nil, :test_data=>"70"}] 
1.9.3-p551 :003 > Timetwister.parse('Aug. 3, 1886')
 => [{:original_string=>"Aug. 3, 1886", :index_dates=>[1886], :date_start=>"1886-08-03", :date_end=>"1886-08-03", :date_start_full=>"1886-08-03", :date_end_full=>"1886-08-03", :inclusive_range=>nil, :certainty=>nil, :test_data=>"200"}] 

As such, there needs to be a check for ordinal values, and either replace them in-line ('first' -> '1') or remove the ordinality ('1st' -> '1')