Fix minor issues related to attributes throughout the code
Capacity gossip is now gossiped per store, not by group.

This should be sufficient for the beta stages. When more stores
are added, then switch to gossiping capacity through groups,
so that only the top N will be gossiped. This will lower the gossip
and storage load on all machines.
Simple, linear implementation of a store finder for allocation decisions

Store now tracks gossip keys of store capacity, and scans the capacity gossip
for attribute matches when they need to make allocation decisions. A simple,
linear implementation that has a straightforward way to implement garbage
collection of capacity keys, but faster methods can be employed later. It
is likely sufficient as long as it is sufficient for all stores to gossip
capacity, but will need to be reevaluated when capacity is gossiped through
groups.
Address Spencer's PR comments

Changes integer base on capacity gossip keys and adds TODO
for possible speedup.
Embeds a StoreFinder into Store

In response to Spencer's PR comments, rather than increasing
the length of storage/store by defining store finder methods on Store itself.
Merge pull request #187 from embark/embark/rebalancing

Simple StoreFinder and new capacity gossiping