Proposed patch to help in extending LittleProxy

Primary purpose of this commit was to extend LittleProxy through a dependent project. The project extends DefaultHttpProxyServer. Converted many fields from private to protected. Wrapped all object instantiations in a protected method. Both of these changes allow for subclasses to have access to state data and return other subclasses for the rest of the implementation. Added a few protected void callbacks in important parts of the code so that the subclass can do things like track the request flow. Also needed access to the default ProxyUtils.via, so made a get/set on that. There was also a bug where it appended a period after the Via minor version instead of a space.