fix bug 147, add .swp to gitignore

fix bug #147: print to PDF clear text on schematic items

I think the result is because 
float FontScale is calculated from screenDpi/printerDpi, and sometimes becomes 0.
the font size will change if multiply/divide by 0
simply add a if condition to avoid this
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also, add .swp in gitignore to avoid add vim temp. file being add into repository