Fix "Branch name doesn't conform to GIT standards"

[EricCousineau-TRI]

Resolves #74

Uses suggestion from @jan-hudec, using urllib, but keeping (at least some) backwards compatibility with existing source transforms.

Can supersede #7

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This change is 

[EricCousineau-TRI]

Difference from original error in #74:

$ git fetch upstream 
searching for changes
no changes found
From hg::ssh://hg@bitbucket.org/osrf/sdformat
   501ada1..e50fb23  branches/pose_dom -> upstream/branches/pose_dom
 * [new tag]         sdformat3-mentor2_3.99.99%2Bmentor2v1.0 -> sdformat3-mentor2_3.99.99%2Bmentor2v1.0
 * [new tag]         sdformat4-mentor2_4.99.99%2Bmentor2v1.0 -> sdformat4-mentor2_4.99.99%2Bmentor2v1.0
 * [new tag]         sdformat4_4.0.0%7Epre1 -> sdformat4_4.0.0%7Epre1
 * [new tag]         sdformat4_4.0.0%7Epre2 -> sdformat4_4.0.0%7Epre2
 * [new tag]         sdformat4_4.0.0%7Epre3 -> sdformat4_4.0.0%7Epre3
 * [new tag]         sdformat4_4.3.0%7Epre1 -> sdformat4_4.3.0%7Epre1
 * [new tag]         sdformat4_4.3.0%7Epre2 -> sdformat4_4.3.0%7Epre2
 * [new tag]         sdformat5_5.0.0%7Ebeta1 -> sdformat5_5.0.0%7Ebeta1
 * [new tag]         sdformat5_5.0.0%7Ebeta2 -> sdformat5_5.0.0%7Ebeta2
 * [new tag]         sdformat6_6.0.0%7Epre1 -> sdformat6_6.0.0%7Epre1
 * [new tag]         sdformat8_8.0.0%7Epre1 -> sdformat8_8.0.0%7Epre1

[felipec]

It is not clear exactly what the patch does, you need to write more description into the commit message.

Also, you should write a simple test case that reproduces the problem, so that we can add it to the test suite and make sure it's working correctly.

And you please rebase and send the pull request again?