Add a custom Curl probe.

Update curl output to support all timers (see bellow).

Add new configuration option 'write_out'.
This option designate which timer information we want to get from curl output.

'write_out' can take one of the following values:

* time_namelookup
* time_redirect
* time_connect
* time_appconnect
* time_pretransfer
* time_starttransfer
* time_total

Default behaviour is the same than current Curl probe: total - DNS
Merge pull request #24 from jbfavre/feat_add_custom_curl_probe

Add a custom Curl probe.