Catkin-tools-fetch

- handle conflicting branches in the `package.xml` files.

Main changes
- `update` and `fetch` now run in parallel to speed up git cloning (arguments `-j` and `--num_threads` control number of threads spawned by the tool)
- new printing mechanism that supports colors and prints a status message to always see that the code is actually doing something (disabled by argument `--no_status`)

- fix a bug, where if the user specifies packages to be downloaded, their dependencies were never downloaded.
- allow cloning a specific branch specified in `<export>` tag in `package.xml`. See Readme for more info.

Example of using `branch` in `package.xml`:
xml
<?xml version="1.0"?>
<package>
...
<export>
<git_url target="depth_clustering" url="gitgithub.com:niosus/catkin_tools_fetch.git" branch="master" />
</export>
</package>

- new verbs `deps` and `dependencies`
- old verb `fetch` deprecated, but still works in this version
- new sub-verbs `fetch` and `update`
- `update` sub-verb to update all repositories to the latest version from git

- fix running with python 3

- If `package.xml` dependencies contain spaces, they will be stripped, but a warning will be shown to the user.