notification: make icon visible

The previous notification icon was a black exclamation point, which at least on modern versions of Android is invisible.  Following the [iconography guidelines](http://developer.android.com/design/style/iconography.html) and using the [android-ui-utils notification icon generator](http://android-ui-utils.googlecode.com/hg/asset-studio/dist/icons-notification.html) I've made a simple clock icon that in addition to being visible should also fit in reasonably well with other icons.

I added the Inkscape svg I used to create the icon to a new `misc/` folder because I wasn't sure where else would be good.