embed-go filename generation breaks for relative paths

[cortesi]

File names for embedded boxes are generated with the following code:

boxFilename := strings.Replace(boxname, "/", "-", -1)
boxFilename = strings.Replace(boxFilename, "..", "back", -1)
boxFilename = boxFilename + `.rice-box.go`

This breaks for relative paths. For instance, the path "./static" will become ".-static.rice-box.go". Note the leading period. Hidden files are ignored by go build, and the box is not included in the binary. The method also breaks in a variety of rare corner cases, like the presence of directories named "back".

This problem is a complicating factor in issue #66 - the example I gave there is actually this bug, not a cross-compilation issue.

I have a pull request inbound that fixes this problem.