Updates documentation in README.md for insert/update/delete and adds a few tests for values helper

EDIT: I made a mistake, read [my last comment](https://github.com/jkk/honeysql/pull/19#issuecomment-37092418) first please.


The formatting for values when passing in a hash-map is weird in 0.4.3:

```clj
=> (require '[honeysql.helpers :as hh] '[honeysql.core :as hsql])

;; This makes sense and is good:
=> (hsql/format (-> (hh/columns :foo :bar)
                    (hh/values [["foo1" "bar1"] ["foo2" "bar2"]])))
["(foo, bar) VALUES (?, ?), (?, ?)" "foo1" "bar1" "foo2" "bar2"]

;; This is broken:
=> (hsql/format (hh/values {:foo "foo" :bar "bar"}))
["VALUES (foo, ?), (bar, ?)" "foo" "bar"]
```

What would be correct is this:
```clj
=> (hsql/format (hh/values {:foo "foo" :bar "bar"}))
["(foo, bar) VALUES (?, ?)" "foo" "bar"]
```

...and [based on the code](https://github.com/jkk/honeysql/blob/master/src/honeysql/format.clj#L372-L375), it seems like that was the intention.  However, [`sequential?`](https://github.com/jkk/honeysql/blob/master/src/honeysql/format.clj#L369) returns true for the first values in hash-map because they convert to MapEntries, which are sequential...d'oh.

Anyways, I've fixed this and added tests.  I've also added documentation in the README.md for using the new insert, update, and delete helpers.