Remove some unnecessary "begin" keywords

`begin` [is a code smell](http://youtu.be/qEHn46YtRM8?t=27m25s), so I've looked into Resque's usage of this keyword.

When `begin` is applied to an entire method, it can be replaced by the idiomatic method-level exception clause. This is what I've done in this pull request.

There are 31 remaining instances of `begin` (8 in lib, 23 in test) and most of them are smells that indicate refactoring should be done. But I'll leave that for future pull requests.