support ampersands

[alexduryee]

Currently, ampersands/'and' are not well-supported for dates of the form "Dec. 10 & 11, 1855". They should be properly supported.

1.9.3-p551 :011 > Timetwister.parse('Dec. 10 & 11 1855')
 => [{:original_string=>"Dec. 10 & 11 1855", :index_dates=>[], :date_start=>nil, :date_end=>nil, :date_start_full=>nil, :date_end_full=>nil, :inclusive_range=>nil, :certainty=>nil}] 
1.9.3-p551 :012 > Timetwister.parse('Dec. 10 and 11 1855')
 => [{:original_string=>"Dec. 10", :index_dates=>[], :date_start=>nil, :date_end=>nil, :date_start_full=>nil, :date_end_full=>nil, :inclusive_range=>nil, :certainty=>nil}, {:original_string=>"11 1855", :index_dates=>[], :date_start=>nil, :date_end=>nil, :date_start_full=>nil, :date_end_full=>nil, :inclusive_range=>nil, :certainty=>nil}]