Adding git repo ability for templates

Adding the ability for `motion create` to specify a remote git repo for templates, which it will then pull down locally on demand.

e.g.
```
$ motion create --template=https://github.com/IconoclastLabs/smart-ios.git test
```
*OR*
```
$ motion create --template=git@github.com:IconoclastLabs/MotionTemplate.git myapp
```

Also, we added documentation to the Readme on how to fork/modify the `create` command for future devs.