Move "github" package from openwhisk to openwhisk-catalog

Issue #4 is a prereq.

Move sample "github" in current repo./directory:

```
/openwhisk-openwhisk/catalog/github
```

to the "openwhisk-catalog" repo. under directory structure:

```
/openwhisk-catalog/packages/github
```

including action files

```
webhook.js
```

Update install script (in /openwhisk-catalog/packages directory, after Issue #4 is completed) :

```
installGit.sh
```
