Add error message support to the railgun code

This code was lost in the transition when the meterpreter source was
removed from the metasploit-framework source. I'm pulling this in by
request of @dmaloney-r7 who originally requested this code be inculded
as part of https://github.com/rapid7/metasploit-framework/pull/740

I added an extra bit of code to free up memory that is allocated by the
call to FormatMessage and forced the ASCII-version (FormatMessageA) of
the call.
Remove unused sniffer config, add PssdkVersion setting

* Rather than having various places where the PSSDK path is set scattered
  through the solution I created a property which contains the version.
  This means we can update that version property in the project and it'll
  take effect everywhere it's required.
* Removed debug and release configurations from ext_server_sniffer.
Updated build documentation

Added more detail around using the appropriate configurations depending on
whether you have access to the appropriate SDKs.
Build status and documnetation update

* Added the build status notification to the top of the README.
* Updated the order of details in the docs (put Linux build after Windows).
* Updated the details about where this repo came from.

Note: this PR should be merged AFTER #20 for ease of merging.
Add posix temp folder to gitignore
Land #24, @OJ's patch to add posix depends .gitignore
Land #20, update docs and project files.

Tested by merging and running a build, as proscribed by the README.md.
12 succeeded, 0 dailed, 2 skipped.
Land #22, more doc and proj updates

Again, tested with a successful run of the build.
Don't lie in the README.md
Land #26, Railgun error messages

See rapid7/metasploit-framework#2443 and rapid7/metasploit-framework#740
Fix railgun multi functionality

The main issue with things being broken was because the calling
convention was not defaulting to "stdapi" and hence the call would
fail. Adding the default fixed it.

While fixing this, I brought the mulit-call functionality up to speed
with the error message functionality, so the calls all return the
properly formatted error message.
Land #28 OJ's Fix railgun multi call functionality
Update the build icon URL

Now that @bturner-r7 has got the CI going on metasploit.com the build
icon needs to point to the new server.
Correct links for source and framework
Land #30, update README.md