no truncating when the expression is a bare symbol, i.e. not an expressi...

...on. closes #19. related to hadley/dplyr#705

A test. @hadley not sure you want to add test suite for lazyeval. 

```
test_that( "auto_name does not truncate symbols (#19)", {
   x <- names(auto_name(lazy_dots(AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA)))
   expect_false( grepl( "[.]", x ) )
})
```