Compatibility with tbl_df from dplyr

[dgromer]

This PR adds compatibility for data frames created with data_frame from the dplyr package to the ezANOVA function.
Right now, when passing a tbl_df as the data argument, the function returns the following error:

Error in ezANOVA_main(data = data, dv = dv, wid = wid, within = within,  : 
  "dv" must be numeric.

This is caused by

if(!is.numeric(data[,names(data)==dv])){

(line 185 in ez-internal.R)

Whereas [.data.frame returns a vector that can be passed to is.numeric, [.tbl_df retuns a single column data frame that returns FALSE on is.numeric.

Minimal example:

dplyr::data_frame(id = 1:15, group = rep(letters[1:3], each = 5), score = runif(15)) %>% 
  ez::ezANOVA(dv = score, wid = id, between = group)