add auto_type_convert option for parser.rb

## summary

We have a problem to analyzing int/float columns with TreasureData or mongoDB which parsed from file with in_tail.
I expect `123` to be integer, `123.45` to be float value but in_tail makes them string type.

On using `fluent-plugin-typecast`, It could convert types by manually.
It is not cool to convert type (cast) by manually. it have to update whenever log format has changed.
it takes much cost of maintenance.

Thus, I have add auto type conversion option for `parser.rb`.

## configuration sample

```
<source>
  type tail
  format ltsv
  time_format %d/%b/%Y:%H:%M:%S %z
  path /var/log/httpd/access_log
  tag debug.apache.access
  auto_type_convert yes
</source>
```
## sample results

the difference seems in status, size, response_time.

```diff
$ diff -u before.json after.json
--- before.json	2013-07-25 22:46:45.000000000 +0900
+++ after.json	2013-07-25 22:47:09.000000000 +0900
@@ -1,3 +1,2 @@
-{"domain":"127.0.0.1","host":"127.0.0.1","server":"127.0.0.1","ident":"-","user":"-","method":"GET","path":"/","protocol":"HTTP/1.1","status":"404","size":"198","referer":"-","agent":"Mozilla","response_time":"398","cookie":"-","set_cookie":"-"}
-
+{"domain":"127.0.0.1","host":"127.0.0.1","server":"127.0.0.1","ident":"-","user":"-","method":"GET","path":"/","protocol":"HTTP/1.1","status":404,"size":198,"referer":"-","agent":"Mozilla","response_time":398,"cookie":"-","set_cookie":"-"}
```