Allowing for small arithmetic errors when calculating column widths

I found another bug in the table width calculation algorithm.

In some cases arithmetic errors lead to errors.

Testcase to illustrate the problem
```` ruby
    it "table widths should be correctly calculated even for column widths with lots of decimals", :focus do
      data=[["a", "b ", "c ", "d", "e", "f", "g", "h", "i", "j", "k", "l"],
            [{:content=>"Foobar", :colspan=>12}]
          ]
      #we need values with lots of decimals so that arithmetic errors will occur
      #the values are not arbitrary but where found converting mm to pdf pt
      column_widths=[137, 40, 40, 54.69291338582678, 54.69291338582678, 54.69291338582678, 54.69291338582678, 54.69291338582678, 54.69291338582678, 54.69291338582678, 54.69291338582678, 54.69291338582678]
      pdf = Prawn::Document.new({:page_size => 'A4', :page_layout => :landscape})
      table = Prawn::Table.new data, pdf, :column_widths => column_widths
      table.column_widths.should == column_widths
    end
````

Output of current master
```` ruby
1) Prawn::Table You can explicitly set the column widths and use a colspan > 1 table widths should be correctly calculated even for column widths with lots of decimals
     Failure/Error: table.column_widths.should == column_widths
       expected: [137, 40, 40, 54.69291338582678, 54.69291338582678, 54.69291338582678, 54.69291338582678, 54.69291338582678, 54.69291338582678, 54.69291338582678, 54.69291338582678, 54.69291338582678]
            got: [59.10301837270342, 59.10301837270342, 59.10301837270342, 59.10301837270342, 59.10301837270342, 59.10301837270342, 59.10301837270342, 59.10301837270342, 59.10301837270342, 59.10301837270342, 59.10301837270342, 59.10301837270342] (using ==)
     # ./spec/table_spec.rb:59:in `block (3 levels) in <top (required)>'
````

This pull request addresses the issue by allowing for an small arithmetic error (e-09) in the two affected algorithms.