Python 3 support

[timonegk]

Some small changes to make urdf_parser_py able to run in python3.

[clalancette]

All of the changes look OK to me (in that they probably won't break anything on python 2), with the possible exception of the addition of .decode(). I've had trouble with that before; have you tested on both python2 and python3 with that in place?

[timonegk]

Even though .decode() would work, I think that providing the encoding='unicode' parameter is the better solution because it is easier to understand. In the background, both are the same (see the lxml source). I will edit my commit in a second.

[eacousineau]

Should we perhaps revive Travis CI for Python3-only testing, if we want to vet PRs to ensure that they don't break anything? (at least until the buildfarm supports that configuration?)

[nzlz]

Hey guys, could you accept this PR? I faced the same issue and had to create a solution like the one posted here. Using the official repo is always more convenient.

I can create the PR to a new branch if this is the issue.

[clalancette]

This looks good to me.

@sloretz unless you have objections here, I'll merge this later today.