release: 0.1.26

This is the last release from 0.1 API cycle.

0.2 cycle starts and API/ABI will change.
build: Starts 0.1.27 development cycle, which will end up in release 0.2.0

From now on, API/ABI will be significantly broken!
Removes EvdServiceValidate and makes EvdService use EvdValidateEnum
Adds new signal 'validate-tls-connection' to EvdService

and renames validate_connection() signal prototype to
signal_validate_connection(), for consistency.
Adds new virtual method connection_rejected() to EvdService
Finishes implementation of connection validation in EvdService
Adds new methods accept_connection() and reject_connection() to EvdService
Removes unnecessary object referencing to EvdService connections during TLS
Improves connection sanity checks in add() method of EvdService
Adds new method get_io_stream_type() to EvdService
Adds annotation and fixes a few coding style issues in EvdHttpMessage
Improves handling of peer object references in EvdPeerManager
Moves static func cleanup_peers_cb up above public methods in EvdPeerManager
Prevents HTTP caching on handshake response in EvdWebTransportServer
Adds set/get_tls_autostart() and set/get_tls_certificate() to EvdConnectionPool

Internal implementation of TLS upgrade is not ready yet.
Implements TLS upgrade in EvdConnectionPool when 'tls_autostart' is enabled
build: Removes C89 compliance flag from CFLAGS

Unfortunately, GnuTLS 3.0 headers are not C89 compliant and compilation
is failing. Upgrading to GnuTLS 3.0 is unavoidable at this point.
Delays sending response headers after target file has been opened in EvdWebDir

Sending the response headers before opening the file is buggy because file
can fail to open (i.e, permission denied) but the response headers are already
sent as 200 OK.
Simplifies a bit peer message tracking logic in EvdTransport
Adds new method respond_headers() to EvdWebService

It is actually a split of code from respond_internal() which sends both headers
and content.