Adding specific Popen creation flags on Windows 

Adding specific Popen creation flags on Windows to avoid the disturbing cmd.exe window opening while executing bower commands.

Visual explanation of that was happening before while installing a package : 

![explanation](https://f.cloud.github.com/assets/164834/347256/80513980-9eb5-11e2-97fe-c26cd7dddc1b.png)

When bower finished executing the window was automatically closed.

After applying this patch, sublime-bower behaves exactly like on your presentation gif while run on Windows.