More options from Platform#platform, add Platform#platform_description

I've picked up @sgottreu's commits from #76 and responded to @fnando's feedback:

- return symbols from `Platform#platform`
- use `Platform#platform_description` to return human-friendly string 
- update tests for new platform detection 

Also, I added documentation about customizing those strings.

I had to change one previous test: BlackBerry devices no longer have `other` in their meta info, now they have `blackberry`. 

What do you think?