Add Podspec (address #9)

This adds valid podspec.

Couple of notes:
- Podspec must have LICENSE. It is not possible to have Podspec without LICENSE. I created LICENSE file and copied there license text from `Xtrace.h`. It is BSD 2-Clause, correct?
- Podspec must point to some specific tag. Tag must contain the same version number as `s.version`. So please set a tag on the latest commit like v4.2 and modify Xtrace.podspec so that its `s.source` points to this new tagged commit.
- Now Xtrace.podspec points to April's `v4.1` which has Xtrace files within Xray directory. When you create new tagged commit be sure remove Xray for the `s.source_files = ...` so it would be:

```ruby
s.source_files = 'Xtrace.h', 'Xtrace.mm'
```

After you have new tagged commit it is time to push your podspec to trunk. It is very to accomplish, just follow these simple tutorials - it becomes very easy once you get it for the first time:

http://useyourloaf.com/blog/2014/08/13/creating-a-cocoapod.html
http://guides.cocoapods.org/making/getting-setup-with-trunk.html

I am ready to help. Let me know if it worked for you.