Implement a rebroadcast handler.

This commit implements a rebroadcast handler which deals with
rebroadcasting inventory at a random time interval between 0 and 30
minutes.  It then uses the new rebroadcast logic to ensure transactions
which were submitted via the sendrawtransaction RPC are rebroadcast until
they make it into a block.

Closes #99.
Don't discuss internals in exported func comments.

This commit cleans up and moves a couple of comments in the recent pull
request which implements a rebroadcast handler (#114) in order to avoid
discussing internal state in the exported function comment.

How a function actually accomplishes the stated functionality is not
something that a caller is concerned with.  The details about the internal
state are better handled with comments inside the function body.