Updated percentage stats for overall pools

For example, to find the percentage of rejected shares out of all shares (Accepted, Rejected and Discarded), one route is to divide # of rejects by # of all shares, then multiply by 100 to obtain a percentage.
I've implemented this for both the Reject and Discard column under the Pool Status
Moved percentage calculations to the Difficulty based statistics

Updated based on feedback from @kanoi on the previous commit - https://github.com/MineForeman/MinePeonWebUI/pull/11#issuecomment-20971927
Removed unnecessary </td> tag
Added Difficulty Stale column, count and percentage to the pool status table

Per suggestion from @kanoi : https://github.com/MineForeman/MinePeonWebUI/pull/11#issuecomment-20971927
Merge pull request #11 from joeykrim/patch-4

Updated percentage stats for overall pools