Add `-version` option to get just the version

Adds a `-version` or `--version` option to print just the version of the program for bitcoind, bitcoin-cli and bitcoin-qt (almost every program on linux supports this, I've made the mistake of calling bitcoind with --version multiple times already, before deciding to finally add it).

Also make it that `-help` can be used to display the help (as well as existing `--help`). Up to now, `-help` was the only option that didn't work with either one or two dashes.

Edit: maybe it's a good idea to add license information as well (for example, `ls` does this too) as print the versions for dependency libraries such as OpenSSL, that would fix #1930.
I think it's ok to split out things like the copyright notice or the "this is experimental software" notice. I think the easiest solution for proper newlines is to leave the first two messages seperate and combine the second paragraph into one message and the third paragraph in one message. Then only the two paragraphs need to be properly wordwraped, which is probably supported by some boost function.