Code cleanup

This is a major code cleanup I'd like to propose before tackling issue #72 which is going to introduce new classes and a trait.

Currently the v8js.cc class is rather messy IMHO as it not only holds everything related to our V8Js main class, but also V8Object & V8Function classes as well as all exception classes + v8 initialization.
I've now split all that stuff into seperate files (v8js_class.cc, v8js_v8object_class.cc, v8js_exceptions.cc + non class stuff in v8js_timer.cc & v8js_v8.cc)

Besides I found the similarly named handler functions relating to v8js (php_v8js_XXX) and v8object (php_v8js_v8_XXX) classes rather confusing and hence renamed the latter to v8js_v8object_XXX (i.e. with "object" term in it)

Also we had a huge mess with symbols beginning with "php_v8js_" and just  "v8js_"; e.g. `php_v8js_ctx`, `php_v8js_execute_string` as opposed to `v8js_to_zval`, `v8js_persistent_obj_t` et al.
So I removed all the leading php_ prefixes.

@preillyme @cscott please let me know what you think

cheers
  ~stesie