Implement GtkHeaderBar
Implement GtkCellRendererToggle.

Squashed and fast-forwarded merged from weberc2's fork:
https://github.com/weberc2/gotk3.git

Fixes #28.
Implement SetIconFromFile for GtkWindow

SetIconFromFile allows a user to set the icon for GtkWindow's and
GtkDialogs using a filename.
Prepare for release 0.1.0.
Fix typo in CHANGES.
Reference both created Github issue for a fixed issue.
Remove duplicate example.
Fill out more bindings for gtk.MessageDialog.
Update CHANGES for new MessageDialog additions.
Fix a premature free in Connect.

User data passed to calls to (*glib.Object).Connect is marshaled
through glib as a glib.Value with a pointer type pointing to a Go
reflect.Value of the user data.  Doing this gives C code a pointer to
a Go structure.  Because the glib.Value has a runtime finalizer set to
unset the value, the underlying pointer to the reflect.Value may be
destroyed once the glib.Value goes out of Go's scope.  This change
modifies the closures map to add a pointer to this glib.Value so the
finalizer will not run.
Fix a memory leak in Connect.

This change adds a finalizer notifier for all GClosures created for
Connect.  When the GClosure is finalized, the closure and closure
context is removed from the internal closure map.  Removing the
contexts from this map removes the reflected func and user data from
Go's scope, allowing them to be garbage collected.
KNF
Add Travis-CI support.
Add travis-ci build status to README.
Make all constants typed.

Before, iota was used for grouped constants representing C enums.
This was changed to the following pattern to prevent any issues in
case the C changes (for example, an enum is reordered):

type Thing int

const (
	THING_A Thing = C.THING_A
	THING_B       = C.THING_B
)

However, unlike with iota, this pattern does not set the type Thing
for all consts in the group, and THING_B becomes an untyped constant.
This can lead to bugs where the compiler is unable to catch issues
wehre THING_B is passed as an argument to a function that should not
take a Thing.

Application code may need to be fixed to properly use constants if
previously misused.
Add ALIGN_BASELINE const (since GTK 3.10).
Add 2014 to copyright dates.
Fix a memory leak with IdleAdd.

This change runs the removeClosure function to remove a closure
context from the internal closures map when glib finalizes closures
created for the IdleAdd function.  A similar change was made to stop a
leak in Connect, but remained in IdleAdd.
Prepare for release 0.1.1.
Added TreeModel.IterNext()