Support alias names for bundled templates

[AliSoftware]

This new feature allows us to create alias names for existing templates.

To create an alias template, simply create a text file named <alias_name>.alias whose first line just contains exactly the name of the template to point to. Note: Any text after the first line of this *.alias files is ignored, so the next lines in the file can be used as comments if necessary.

Introducing android-java and swift4 template names

This feature allowed us to:

• Rename the original template android to android-java, for better consistency with the new android-kotlin template
• But still provide an alias to it via the name android, so that using -t android will still work (and not break the setup for existing users of gyro) and use the same template as if we used -t android-java explicitly
• Also provide an alias named swift4 pointing to the template swift3, since the swift3 template is also compatible with Swift 4 so using the alias allows for a more logical name in the invocation of gyro.

This PR also closes #29

[StevenWatremez]

I have a question about alias for custom path. Do you want to authorize creating custom alias for custom templates or not ?

Actually we have two resolution methods for templates. Template.find_by_path(path) and Template.find_by_name(name), this last method implement alias resolution but not the first.

[AliSoftware]

I have a question about alias for custom path. Do you want to authorize creating custom alias for custom templates or not ?

I wondered about that but I don't see how that would work or be useful. I mean, if you have custom templates anyway: they'll only work for your machine / your project because you'll have to have the custom templates you created on the machine to run gyro. So if that's templates that you created yourself, you can give them whatever name you want, right?