Squish `Unapply` implicitNotFound msg into 1 line.

I hit this issue when trying to generate a
[Kapeli Dash](http://kapeli.com/dash) docset for Scalaz
using [this](https://bitbucket.org/inkytonik/mkscaladocset).

Issue: Scaladoc generation first checks to see if the
message is >= 36 characters long - if so, instead of
showing a big whopping message in the HTML it shoves
it into a tag attribute instead with the motivation being
you can hover your mouse over it to read the full message.

However, the generation process will also replace any
newlines with the HTML newline, `<br />`. When it goes
to turn `Unapply`'s `implicitNotFound` message (which is
>= 36 characters long *and* multi-line) it translates
the end of each line into `<br />`, then puts it into a
tag attribute. As far as I know, angle brackets are not
allowed in HTML tag attributes, but Scaladoc generation
will gladly generate it anyways.

My proposed solution is simply - squash it all down into
one line.
Merge pull request #604 from adelbertc/fix-unapply-html

Squish `Unapply` implicitNotFound msg into 1 line. #555