fix bug 147, cmake support message

There are two modification.
First is bug 147:
I found that the base problem is that qucs/schematic.cpp pass float argument by a division of two integer, and doesn't add (float) casting.
After adding casting, the resize ratio will not be zero anymore.
Consider cpp style, maybe modify to static_cast is recommended.

Second is cmake supported
Since I cannot compile qucs/qucs on archlinux by autoconf/configure/make, I use cmake to compile the project.
I add some recommended message in README.md and ignore the recommended build path in .gitignore

The last one is ignore vim temporary file .*.swp file