Document the random graph algorithms.

I've put them under Algorithms, even though all other graph constructors
were under Graph Types. I'm not sure if this is correct.
Merge pull request #62 from pozorvlak/maximal_cliques

Maximal cliques function.
Remove type signatures from random graph docs.
Merge pull request #63 from pozorvlak/doc_random

Document the random graph algorithms.
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
```