HAR support

Changes in this pull request : 

* Obviously, HAR support :)
* Added a new dep, jackson-module-scala (easier deserialization to Scala)
* UI change : dropdown to switch between "HTTP Proxy" mode and "HAR Converter" mode, & display appropriate options (network panel or HAR file chooser) according to which mode is selected
* Minor refactoring :
 * Now able to create RequestElement(s) without a Netty's HTTPRequest
 * Extracted filters and redirection handling to logic to share it with HAR