v5.1

This release features a fixed version of budgie-panel rewritten in Vala.
As well as resolving issues like static sizing and panel
position, the automatic darkening of the panel has been vastly improved.

This release also sees the introduction of a new "BudgiePlugin" API, with
all of the applets now being loaded using libpeas-1.0 and the internal
API. The intent is that in v6 we will have a customisable panel layout,
enabling you to add or remove applets and place them in any order that
you wish.

Note: The reason this is 5.1 is because v5 was borked due to autotools
not liking the vapi generation.