S3 method for microbenchmark

S3 method for microbenchmark class.
Also supports unit param and user defined expression names.

**Basic examples**

```
> pander(b)

--------------------------------------------------------
            expr               min   lq    median   uq  
----------------------------- ----- ----- -------- -----
    pander(data.frame(a =     3.543 4.264  4.822   6.138
"foo\nbar"), keep.line.breaks                            
           = TRUE)                                      

    pander(data.frame(a =     3.415 4.018  4.614   5.504
         "foo\nbar"))                                    
--------------------------------------------------------

Table: Unit: milliseconds (continued below)

 
-------------
 max   neval 
----- -------
11.61   100  

11.5    100  
-------------
```
```
> pander(b, split.tables = Inf)

----------------------------------------------------------------------
            expr               min   lq    median   uq    max   neval 
----------------------------- ----- ----- -------- ----- ----- -------
    pander(data.frame(a =     3.543 4.264  4.822   6.138 11.61   100  
"foo\nbar"), keep.line.breaks                                          
           = TRUE)                                                    

    pander(data.frame(a =     3.415 4.018  4.614   5.504 11.5    100  
         "foo\nbar"))                                                  
----------------------------------------------------------------------

Table: Unit: milliseconds
```
Supports user defined expression labels through `expr.labels`
```
> pander(b, split.tables = Inf, expr.labels = c("First", "Second"))

-----------------------------------------------
 expr   min   lq    median   uq    max   neval 
------ ----- ----- -------- ----- ----- -------
First  3.543 4.264  4.822   6.138 11.61   100  

Second 3.415 4.018  4.614   5.504 11.5    100  
-----------------------------------------------

Table: Unit: milliseconds
```
Supports different units through `unit` param.
```
> pander(b, split.tables = Inf, expr.labels = c("First", "Second"), unit = "s")
----------------------------------------------------------
 expr    min       lq     median     uq      max    neval 
------ -------- -------- -------- -------- ------- -------
First  0.003543 0.004264 0.004822 0.006138 0.01161   100  

Second 0.003415 0.004018 0.004614 0.005504 0.0115    100  
----------------------------------------------------------

Table: Unit: seconds
```