Add windows support

[dpwright]

Windows expects executables to have a .exe file extension, and the lack of
it causes various problems trying to run the plugin.

This commit fixes the filename for protoc on Windows. It doesn't fix the
other main problem with running the plugin on Windows, which is that
building protobuf requires you to be in a MinGW environment and gets a bit
involved. The expectation is that on Windows you'll have to build
protobuf yourself, but once you've done that the plugin should work as
normal.

[dpwright]

I just hacked this in to get it working on Windows... it's a bit nasty-looking and I don't know whether you'd want to include dirty Windows hacks like this in your plugin's main code, but I thought I might as well send you what I'd done in case it comes in handy for anybody using lein-protobuf on Windows.

[cpmcdaniel]

I'm actually working on a patch that does it more directly by allowing you to specify a :protoc-exe path in the project map which will completely bypass the downloading and compiling of the protoc executable. Perhaps this plugin is trying to do too much. Can we not assume the user is capable of installing protoc themselves? What if they already have it installed, or are using it in multiple projects?

I'm not ready to submit the pull request yet. I'm actually having an issue with the regex for the proto import statements. It seems that the double-quotes are being stripped out. Once I get around this, I'll submit.