Add version information to launcher

We now add versioning information to the launcher script. When the
docker container is launched it will log version information prefixed
with 'version'. The version information contains:

- Launcher script version
- Emulator version
- System image version.

For example:

version: Android emulator version 29.1.12.0 (build_id 5830323) (CL:N/A)
version: launch_script: 0+untagged.19.g35b4367.dirty
version: Pkg.Desc=System Image x86_64 with Google Play.
version: Pkg.Revision=8
version: Pkg.Dependencies=emulator#27.1.7
version: AndroidVersion.ApiLevel=28
version: SystemImage.Abi=x86_64
version: SystemImage.TagId=google_apis_playstore
version: SystemImage.TagDisplay=Google Play
version: SystemImage.GpuSupport=true
version: Addon.VendorId=google
version: Addon.VendorDisplay=Google Inc.

This can be used to better assist those that need help. Introduces
a dependency on versioneer that will update the version based on
git and tags. See https://github.com/warner/python-versioneer for
details.

.gitattributes

@@ -0,0 +1 @@
+emu/_version.py export-subst

MANIFEST.in

@@ -0,0 +1,2 @@
+include versioneer.py
+include emu/_version.py

emu/__init__.py

@@ -0,0 +1,4 @@
+
+from ._version import get_versions
+__version__ = get_versions()['version']
+del get_versions