Make .strings file diff-able

Requires also adding the following to your git config:

```
[diff "utf16"]
    textconv = "iconv -f utf-16 -t utf-8"
```
Add gen:strings Rake task that updates strings files
Update Drop-In and UI strings
Update gen:strings Rake task to generate UTF-8 strings files

Makes for easier github diffing
Revise feedback section in README
Merge branch 'feature/paypal-2.7.1'
Update podspec description and summary
Add a new-line at the end of every file to support merchant integrations with GCC_WARN_ABOUT_MISSING_NEWLINE
Merge pull request #84 from braintree/feature/new-line-at-end-of-file

Add a new-line at the end of every file to support merchant integrations with GCC_WARN_ABOUT_MISSING_NEWLINE