Change Session to struct and add simple support for storing generic types in sessions.

This change is partially backwards-incompatible due to Session being changed to struct.

See #427.
Move the REST interface generator from vibe.http.rest to vibe.web.rest. See #427.
Add a simple implementation of registerWebInterface along with an example package. See #427.
Add a warning if neither VibeCustomMain, nor VibeDefaultMain is given - starts the transition phase from VibeCustomMain to VibeDefaultMain.
Fix compilation on DMD 2.063.2 and remove const from Session getter methods.
Don't use "with" + "alias this" (doesn't work on DMD 2.063).
Update change log.
Add a deprecation notice for registerFormInterface.
Allow optional _parameters with default values and enable T->string conversion using T.fronStrimg().
Don't allow -noboundscheck for vibe.d based applications until the API is `@safe`.
Fixed an exception raised with empty json requests.
implement logFatal
Add assertions against using uninitialized MongoCollection instances. Fixes #437.
Merge pull request #441 from gittywithexcitement/master

implement logFatal
Merge pull request #440 from Archytaus/master

Fixed an exception raised with empty json requests.
Fix "mainSourceFile" and add "VibeDefaultMain" to the unittest configuration.
Fix unit test.
Add an example and a notice about interaction with other tasks to sleep(). Fixes #434.
Fix code style in the HTTP server implementation.
Fix WebSocket request processing for IE10/11 (sends "Websocket" instead of "websocket").