Remove dashes from license field

[coldfix]

Hey, this contains a fix for a very small issue encountered when a license text contains a dash. E.g. if the license is "non-free". At the moment, this will generate the following shield:

After the patch:

The same issue probably exists for other fields as well.

[coldfix]

Sorry, probably its better to fix the issue inside shields, I'll look into it.

[coldfix]

Please forgive my ignorance: I just realized, the shields API already supports dashes via --. I amended the commit.

Also escaping other special characters (/$?= etc) now.

If you want, I can squash-rebase the commits.