Keep track of redirect hits and last access time

Resolves Issue #4.

Keeps track of redirect hits and last access time. Both columns are sortable and the hit counter can be reset.

![2014-09-11_00-59-46](https://cloud.githubusercontent.com/assets/53005/4229447/8975d910-3970-11e4-8d91-1f28c173ade5.png)
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![2014-09-11_01-01-40](https://cloud.githubusercontent.com/assets/53005/4229452/c0b5b594-3970-11e4-80d4-13954a1049f6.png)

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This should be thoroughly tested to ensure backwards compatibility with existing Redirects created with an earlier version of WP-Redirects.