A* search algorithm.

The API is based on dijkstra_shortest_path, particularly in that it
takes edge weights as a Vector{D} rather than a function or an edge
annotation.

A* relies on a priority queue, for which I used
Base.Collections.PriorityQueue. The enqueue! and dequeue! methods
conflicted with those of Queue, breaking the BFS tests. Hence I've
wrapped the A* code in a module and exported a wrapper function into the
Graphs namespace.

The test is the "tour of Romania" example from Norvig and Russell's AI book.
Update cliques to initialize pivotdonenbrs

pivotdonenbrs is not in a high enough scope to be seen between when it is defined and when it is referenced the second time.  Without this change I get:

```
ERROR: pivotdonenbrs not defined
 in maximal_cliques at /home/user/.julia/v0.3/Graphs/src/cliques.jl:125
while loading solver.jl, in expression starting on line 38
```
Merge pull request #67 from pozorvlak/maximal_cliques_bug

Maximal cliques bug
Merge pull request #68 from pozorvlak/a_star

A* shortest-path algorithm
Test against Julia releases in CI.

This reverts ea4f4d0; only testing against nightlies made it harder to
track down a couple of build failures that were due to changes in the
Julia compiler.