Fix OmniSharp launch under Windows
Small refactoring

Use the dedicated function to check if we're running on Windows instead
of checking the platform directly.
Fix faulty logic in previous commit

Previous commit is 9b8781322ef3d18430c3da37e9d6b1be3b832f8b
Fixed OmniSharp launch under Windows (again)

We pass shell=True to Popen so that OmniSharp is not started inside a
new visible window under Windows. And since we use shell=True, we pass
the command to execute as a string, as recommended by Python's docs
(also, it won't work when passed as a sequence anyway :) ).
Removed an unused import
Small stylistic change
New wrapper function around subprocess.Popen

New wrapper function around subprocess.Popen that handles stdin
correctly when on Windows (see issue #637)
Merge branch 'fix-omnisharp-launch' of https://github.com/nop00/YouCompleteMe into nop00-fix-omnisharp-launch
Forwarding args for SafePopen to Popen directly