[Feature] Add --pull and --build options

[drrosa]

These options help the user automatically grab all repos and build Gazebo from source when using the --source option.

Examples: The entire install from source can be done all at once gzdev spawn 9 --source --pull --build

Or step by step as shown below:

Set up development environment and dependencies: gzdev spawn 9 --source

Pull all the official source code repositories: gzdev spawn 9 --pull

Compile and install everything from source gzdev spawn 9 --build

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This change is 

[j-rivero]

The code looks good to me.

For some reason, the vcstool is failing to get gazebo every time I try to get gazebo8 or gazebo9 running with --build and --pull.

jrivero@nium gzdev $ (gz-src-install) docker logs gz9_src -f
mkdir: cannot create directory ‘src’: File exists
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   870  100   870    0     0    979      0 --:--:-- --:--:-- --:--:--   978
.......E
=== src/gazebo (hg) ===
pulling from https://bitbucket.org/osrf/gazebo
searching for changes
no changes found
abort: HTTP Error 400: Bad Request
=== src/ign-cmake (hg) ===
pulling from https://bitbucket.org/ignitionrobotics/ign-cmake
searching for changes
no changes found
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
=== src/ign-common (hg) ===
pulling from https://bitbucket.org/ignitionrobotics/ign-common
searching for changes
no changes found
0 files updated, 0 files merged, 0 files removed, 0 files unresolved

[drrosa]

That seems to happen sometimes when vcs is not doing a "fresh" pull. Try rm -rf gzdev/gazebo/src/gazebo/ and then run it again. If you get the same error, then rm -rf gzdev/gazebo/ the entire directory, and that should fix it.