Write files without execution permission

[come-maiz]

Fixes #43

[ottok]

If you want to make the git commit message explain what this actually fixes, you could say something along:

This was detected by Lintian in Debian emitting following warnings:

W: golang-github-emicklei-dot-dev: executable-not-elf-or-script [usr/share/gocode/src/github.com/emicklei/dot/dotx/TestCompositeWithUnusedIOSameGraph.dot]
W: golang-github-emicklei-dot-dev: executable-not-elf-or-script [usr/share/gocode/src/github.com/emicklei/dot/dotx/TestExampleSubsystemExternalGraph.dot]
W: golang-github-emicklei-dot-dev: executable-not-elf-or-script [usr/share/gocode/src/github.com/emicklei/dot/dotx/TestExampleSubsystemSameGraph.dot]
W: golang-github-emicklei-dot-dev: executable-not-elf-or-script [usr/share/gocode/src/github.com/emicklei/dot/dotx/subsystem.dot]
W: golang-github-emicklei-dot-dev: executable-not-elf-or-script [usr/share/gocode/src/github.com/emicklei/dot/dotx/subsystem2.dot]