Implemented basic callback based SSL certificate validation.

Still missing host name validation and this commit contains some log output that will be removed prior to release.

The SSLPeerValidationMode enum is also still subject to change and SSLPeerValidationData isn't very useful, yet.
Implement basic SSL host name verification. See #469.
Use DUB 0.9.21-rc.1 for Travis-CI testing to avoid file order dependent errors.
Fix issue #475: diet doesn't allow boolean attributes without value in HTML5
Add html shortcut as HTML5 doctype
Use OutputContext instead for m_isHTML5 flag
Add assertion message for unimplemented Libevent2EventDriver.watchDirectory.
Use latest DUB for Travis-CI (old release was not working due to shared libphobos).
Silence the SSL API.

Also implements support for the permissive validation modes and
silences log output during certificate validation.
Disable the code that loads the system certificate store.

This was far from working cross-platform, so instead of trying to use the system's store, the user now has to supply the set of trusted certificates. At a later time, vibe.d might possibly distribute its own trusted store (e.g. based on Mozilla's one).
Improve HTTPClient's certificate validation handling.

By default, no validation is done to keep backwards compatibility until a proper trusted certificate store is usable by default. At the same time, a user callback can be set to change the validation mode and to add trusted certificates manually.
Add a note about the location of the system's CA store on Linux.
Merge branch 'sslverify'
Remove the RedBlackTree workaround for DMD 2.064 compile errors (fixed through DUB instead).
Update change log.
Add compileDietString() and some basic unit tests for the Diet compiler.
Handle Diet includes as separate mixins().

This commit prepares for support of string templates, originally proposed by Stefan Koch (Uplink_Coder) in #482.
Implement string includes for Diet templates. See #482.
Redo pull #516 without compiler errors (dietStringParser signature improvement).

Fixes #516.
Name Diet string includes like their string expression.