Add project license

[bdemers]

I tried to create/deploy the 2.3.2 on webjars.org and failed to do a missing license file:

Failed!
All attempts to determine an acceptable license have been exhausted. The bower.json file did not contain a spec-compliant license definition and the license could not be determined by trolling through the source repo: https://github.com/Atmosphere/bower-atmosphere
The acceptable open source software licenses are at: https://bintray.com/docs/api/#_footnote_1
The provided licenses were: 
This problem will likely need to be resolved by working with the library maintainers directly.
If you feel you have reached this failure in error, please file an issue: https://github.com/webjars/webjars/issues

Got Bower info
Determined Artifact Name: atmosphere
Starting Deploy
Deploy!

Dropping the license file in should do the trick:
https://github.com/Atmosphere/atmosphere-javascript/blob/master/LICENSE