make installation of addin easier and enable on mac also

[fzumstein]

see also #853

[hydrosquall]

Presently unable to install the add-in on Mac osSierra 10.12.5 with Excel 2016. When I try to install the adding by going to

/Users/cameron/Environments/xlwings/lib/python3.6/site-packages/xlwings/addin/xlwings.xlam as directed by xlwings addin install, and then attempt to add a reference to xlwings, I get a complaint that "Requested type library or wizard is not a VBA project". As a result, I can't even get the hello world example to work.

[fzumstein]

First step is to go to Tools > Excel addins and then browse to your path and select the xlwings.xlam, did you do that? Does the ribbon show correctly?

Then you can do xlwings quickstart ... and it should already have the reference set. If you set it manually, make sure to select the correct project in your VBA editor before clicking on add reference.

[hydrosquall]

It works properly when I ran xlwings quickstart later, since the addin has been installed. I think that maybe the issue was the xlwings quickstart didn't work if the addin hadn't been installed yet.

[fzumstein]

Change installation from semi-hardcoded %APPDATA%\Microsoft\Excel\XLSTART to app.api.StartupPath and app.api.startup_path.get()

[fzumstein]

#1316