Add initial podspec for core client library

Also add license.
Update README.md based on base `pod lib create` template
Do minor updates for CocoaPods, pods, and Podfile
Ensure Travis environment has latest CocoaPods

http://docs.travis-ci.com/user/common-build-problems/#Mac%3A-Errors-running-CocoaPods
Tweak Podfile excerpt in README.md [ci skip]
Merge pull request #15 from 3dna/peng/podspec

Add Podspec. Includes minor CocoaPods-related updates.