Hyphening support for splitting cells implementation

I have added support for using hyphening when splitting large cells. It is implemented by using hyphenation algorithms from [koRpus package](http://cran.r-project.org/web/packages/koRpus/index.html).
Generally it follows all the same rules as relative split of large cells (like in #90), just when use.hyphening option is enabled, it will try to split.words using line breaks also.
_For example_
```
d> df <- data.frame(a="Machiavellianism", b="Hypervitaminosis")
d> pander(df, split.cells=10)

---------------------------------
       a                b        
---------------- ----------------
Machiavellianism Hypervitaminosis
---------------------------------
d> pander(df, split.cells=10, use.hyphening = TRUE)

-------------------
    a         b    
--------- ---------
 Machi-   Hypervit-
avellian- aminosis 
   ism             
-------------------
```
_More general examples_
```
d> text1<- "A named character vector to define replacements for koRpus’ internal headline and paragraph tags."
d> text2 <- "Caseless matching does not make much sense for bytes in a multibyte locale, and you should expect it only to work for ASCII characters"
d> y <- c(text1, text2, "10000000000", "1234567890", text2, "Pandoc Package")
d> y <- matrix(y, ncol = 3, nrow = 2)
d> pander(y, use.hyphening = TRUE, split.table = Inf, split.cells = 10)

---------- ----------- ----------
 A named   10000000000  Caseless 
 charac-                matching 
 ter vec-               does not 
tor to de-             make much 
 fine for              sense for 
 koRpus’                bytes in 
 internal               a multi- 
 headline               byte lo- 
and para-              cale, and 
  graph                   you    
  tags.                should ex-
                        pect on- 
                         ly to   
                        work for 
                         ASCII   
                        charac-  
                          ters   

 Caseless  1234567890    Pandoc  
 matching               Package  
 does not                        
make much                        
sense for                        
 bytes in                        
 a multi-                        
 byte lo-                        
cale, and                        
   you                           
should ex-                       
 pect on-                        
  ly to                          
 work for                        
  ASCII                          
 charac-                         
   ters                          
---------- ----------- ----------

d> pander(y, use.hyphening = TRUE, split.table = Inf, split.cells = 20)

-------------------- ----------- --------------------
 A named character   10000000000  Caseless matching  
vector to define re-              does not make much 
 placements koRpus’               sense for bytes in 
 internal headline                 a multibyte lo-   
and paragraph tags.                 cale, and you    
                                 should expect it on-
                                  ly work for ASCII  
                                      characters     

 Caseless matching   1234567890     Pandoc Package   
 does not make much                                  
 sense for bytes in                                  
  a multibyte lo-                                    
   cale, and you                                     
should expect it on-                                 
 ly work for ASCII                                   
     characters                                      
-------------------- ----------- --------------------