`[`, `$`, etc. for unitted_tbl_dfs

[appling]

ugh to all of this:

library(dplyr)
library(unitted)
> tbl <- u(data_frame(a=1:4, b=5:8), c('A','B'))
> tbl
Source: local data frame [4 x 2]

      a     b
U     A     B
  (dbl) (dbl)
1     1     5
2     2     6
3     3     7
4     4     8
> tbl$a
Error in unitbundle(units) : 
  invalid class for units specification - must be data.frame, character, NA, unitbundle, or missing
> tbl['a']
Error in unitted(vx, new_units) : 
  Number of units must equal number of data.frame columns
> tbl[,'a']
Error in unitted(vx, new_units) : 
  Number of units must equal number of data.frame columns