Re-organize / Rewrite "Double spend" subsection

A lot of re-organization and rewriting here. I felt the general structure of this part could be improved. I've merged both double spend texts into a single one with the following (implicit) structure:

* Concise summary of the problem and the solution
* Explanation about what is a double spend
* Risk accessment for 0 - 1 - 2 - 6 confirmation transactions
* Other risk analysis methods (and reasons to use them)
  * Detecting previous double-spends
  * Checking transaction fees
  * Detecting forks
  * Human intelligence

Some other major changes:

* Begin with a concise summary of the problem and the solution.
* Provide more explanation about what is a double spend.
* Mention how transaction fees can be used affect and detect double spending.
* Mention average 10 minutes time required for first confirmation.
* Mention high-value transactions as a reason to do risk analysis.
* Move "network alerts" text to an HTML comment in the "P2P Network" subsection.
* Drop direct promotion for blockchain.info.
* Shorten texts as much as possible.

All in all, that's a suggestion. I'm fine if this pull req isn't merged in a short time frame, if many changes need to be discussed / improved or if we choose not to merge it for any reason.