geo_join variable matching

[antaldaniel]

First of all congratulations for this great package. I was a bit struggling to find out why I could not geo-match my data. I think that it is a bit unfortunate that there is no exact longitude and latitude variable matching in the geo_join function.

I made a small amendment in the documentation of the function. However, I think that a clearer matching would be more appropriate, given that Google any many other APIs use by default lng for longitudes.

` states <- data_frame(state = state.name,
lng = state.center$x,
lat = state.center$y)

s1 <- rename(states, state1 = state)
s2 <- rename(states, state2 = state)

pairs <- s1 %>%
geo_inner_join(s2, by = c("lng", "lat"),
max_dist = 200) %>%
filter(state1 != state2)
`

Another issue that makes the whole code very difficult to read and amend that only a very few functions are directly imported. For example, in geo_join dplyr::data_frame and dplyr::common_by are not referenced or imported. It is not logical to me why the above modification should not work. Given that in geo-matching lat.x must strictly match with lat.y and lon.x with lon.y it would be preferable to clearly pass on these arguments, for example, like by.lat = c("lat", "lat") and by.lon = c("lon", "lng")