Send only one Cookie header

The use of appendHeader was causing a header to be sent per cookie,
instead of a semicolon delimited list of cookies sent using a single
header.
Add changelog entry for cookie header fix.
Updated cookie header combining to be optional.

Multiple Cookie headers is not actually a bug, but some servers fail to
correctly parse them.
Significant v0.6.0 changes; more after the jump.

Added new expectContinue boolean option to auto-add `Expect: 100-continue` header for requests
with an entity body if no `Expect:` header is already assigned. This option is enabled by default.

MessageParser moved into main Artax namespace, PECL HTTP parser removed

Internal naming, formatting, documentation and minor bugfixes

Amp dependency replaced by new Alert library
Cookie headers now combined by default
Merge branch 'v0.6.0'
Removed test file that should never have been committed
Updates in preparation for v0.6.0
Fixed broken cert-updater progress display
Updated examples ahead of v0.6.0