New S3 method for CrossTable

I completely rewrote S3 method for CrossTable. I used some ideas from `xtable` package. Now the solution seems more robust (I tasted more example than before). There is one decision we need to make though. I set `split.tables` to Inf [here](https://github.com/Rapporter/pander/blob/873c795c50ac3e1ee98a01a425e4e94f3012dd74/R/S3.R#L878). 
It is needed because table has not `rownames`, so if splitting happens there is no `rownames` for next tables. What do you think? Here is an example,
**With split.tables = Inf**
```
> pander(ct1)

---------------------------------------------------------------------
   &nbsp;     0-39g/day     40-79      80-119       120+      Total  
------------ ----------- ----------- ----------- ----------- --------
**25-34**\    &nbsp;\     &nbsp;\     &nbsp;\     &nbsp;\    &nbsp;\ 
    N\          4\          4\          3\          4\        15\    
Chi-square\   0.0016\     0.0016\     0.0938\     0.0494\      \     
Column(%)\   17.3913%\   17.3913%\   14.2857%\   19.0476%\      \    
  Total(%)     4.5455%     4.5455%     3.4091%     4.5455%           

**35-44**\    &nbsp;\     &nbsp;\     &nbsp;\     &nbsp;\    &nbsp;\ 
    N\          4\          4\          4\          3\        15\    
Chi-square\   0.0016\     0.0016\     0.0494\     0.0938\      \     
Column(%)\   17.3913%\   17.3913%\   19.0476%\   14.2857%\      \    
  Total(%)     4.5455%     4.5455%     4.5455%     3.4091%           

**45-54**\    &nbsp;\     &nbsp;\     &nbsp;\     &nbsp;\    &nbsp;\ 
    N\          4\          4\          4\          4\        16\    
Chi-square\   0.0079\     0.0079\     0.0087\     0.0087\      \     
Column(%)\   17.3913%\   17.3913%\   19.0476%\   19.0476%\      \    
  Total(%)     4.5455%     4.5455%     4.5455%     4.5455%           

**55-64**\    &nbsp;\     &nbsp;\     &nbsp;\     &nbsp;\    &nbsp;\ 
    N\          4\          4\          4\          4\        16\    
Chi-square\   0.0079\     0.0079\     0.0087\     0.0087\      \     
Column(%)\   17.3913%\   17.3913%\   19.0476%\   19.0476%\      \    
  Total(%)     4.5455%     4.5455%     4.5455%     4.5455%           

**65-74**\    &nbsp;\     &nbsp;\     &nbsp;\     &nbsp;\    &nbsp;\ 
    N\          4\          3\          4\          4\        15\    
Chi-square\   0.0016\     0.2161\     0.0494\     0.0494\      \     
Column(%)\   17.3913%\   13.0435%\   19.0476%\   19.0476%\      \    
  Total(%)     4.5455%     3.4091%     4.5455%     4.5455%           

 **75+**\     &nbsp;\     &nbsp;\     &nbsp;\     &nbsp;\    &nbsp;\ 
    N\          3\          4\          2\          2\        11\    
Chi-square\   0.0054\     0.4402\     0.1488\     0.1488\      \     
Column(%)\   13.0435%\   17.3913%\   9.5238%\    9.5238%\       \    
  Total(%)     3.4091%     4.5455%     2.2727%     2.2727%           

   Total\       23\         23\         21\         21\        88\   
              26.1364%    26.1364%    23.8636%    23.8636%           
---------------------------------------------------------------------
```
**Without split.tables = Inf**
```
> pander(ct1)

------------------------------------------------------------
   &nbsp;     0-39g/day     40-79      80-119       120+    
------------ ----------- ----------- ----------- -----------
**25-34**\    &nbsp;\     &nbsp;\     &nbsp;\     &nbsp;\   
    N\          4\          4\          3\          4\      
Chi-square\   0.0016\     0.0016\     0.0938\     0.0494\   
Column(%)\   17.3913%\   17.3913%\   14.2857%\   19.0476%\  
  Total(%)     4.5455%     4.5455%     3.4091%     4.5455%  

**35-44**\    &nbsp;\     &nbsp;\     &nbsp;\     &nbsp;\   
    N\          4\          4\          4\          3\      
Chi-square\   0.0016\     0.0016\     0.0494\     0.0938\   
Column(%)\   17.3913%\   17.3913%\   19.0476%\   14.2857%\  
  Total(%)     4.5455%     4.5455%     4.5455%     3.4091%  

**45-54**\    &nbsp;\     &nbsp;\     &nbsp;\     &nbsp;\   
    N\          4\          4\          4\          4\      
Chi-square\   0.0079\     0.0079\     0.0087\     0.0087\   
Column(%)\   17.3913%\   17.3913%\   19.0476%\   19.0476%\  
  Total(%)     4.5455%     4.5455%     4.5455%     4.5455%  

**55-64**\    &nbsp;\     &nbsp;\     &nbsp;\     &nbsp;\   
    N\          4\          4\          4\          4\      
Chi-square\   0.0079\     0.0079\     0.0087\     0.0087\   
Column(%)\   17.3913%\   17.3913%\   19.0476%\   19.0476%\  
  Total(%)     4.5455%     4.5455%     4.5455%     4.5455%  

**65-74**\    &nbsp;\     &nbsp;\     &nbsp;\     &nbsp;\   
    N\          4\          3\          4\          4\      
Chi-square\   0.0016\     0.2161\     0.0494\     0.0494\   
Column(%)\   17.3913%\   13.0435%\   19.0476%\   19.0476%\  
  Total(%)     4.5455%     3.4091%     4.5455%     4.5455%  

 **75+**\     &nbsp;\     &nbsp;\     &nbsp;\     &nbsp;\   
    N\          3\          4\          2\          2\      
Chi-square\   0.0054\     0.4402\     0.1488\     0.1488\   
Column(%)\   13.0435%\   17.3913%\   9.5238%\    9.5238%\   
  Total(%)     3.4091%     4.5455%     2.2727%     2.2727%  

   Total\       23\         23\         21\         21\     
              26.1364%    26.1364%    23.8636%    23.8636%  
------------------------------------------------------------

Table: Table continues below

 
--------
 Total  
--------
&nbsp;\ 
 15\    
  \     
   \    

&nbsp;\ 
 15\    
  \     
   \    

&nbsp;\ 
 16\    
  \     
   \    

&nbsp;\ 
 16\    
  \     
   \    

&nbsp;\ 
 15\    
  \     
   \    

&nbsp;\ 
 11\    
  \     
   \    

  88\   
--------
```