Allow build and start on MINGW (no cygwin required)

[domoran]

Disclaimer: I read in the contribution guidelines, that you do not want contributions to "scripts" ...

Anyway I thought I would add that a windows build does also work without cygwin installed (which is kind of a 'heavy' environment).

I found that an installed GIT is sufficient to build LightTable, since it includes a "bash" shell. Therefore I added inside the build and start scripts the check for the MINGW environment and put another chapter in the documentation for people, that do not want to install cygwin.

[sbauer322]

I'll be able to give it a try in within the next week or so, but it sounds promising!

@LightTable/maintainers - if possible, I would prefer to have multiple people test this out prior to merging.

[sbauer322]

I will have access to a Windows 8.1 machine tomorrow night to test this.

[sbauer322]

Seems I run into a problem with an error when attempting to run script/build.sh with this PR:

script/build.sh: line 25: node_modules/.bin/grunt: No such file or directory

Briefly poking at line 25 and then looking in the contents of deploy/electron/node_modules/.bin/ suggests that there is indeed no file named "grunt"... there is a "grunt.cmd" though. Using npm 3.10.10 and node 6.9.5.

Aside from the error above, there are a couple of other minor things:

• Could you make it more even more obvious that you would need to use the lein script and not the .bat... and also that the script must be in the path (even if the bat is on the path). I managed to overlook the difference and got stuck on it not working with the .bat for a bit.
• It seems that part of the changes to build-app.sh and light.sh is that every line now ends in a whitespace. Could you fix this?
• script/build-app.sh also appears to use cygwin to zip up Light Table. If so, we would likely need the same for MINGW. If you search for cygdrive, you should be able to locate the section.

Overall, I think it just has a couple problems but should be in a good state soon enough. Thank you @domoran!

EDIT: Had to manually delete contents of deploy/node_modules in order to resolve my problem.