Master branch README.md is out of date!

Added clear and clearAll functions and onClear callback for android on Java-side and fixed Problem with manually cleared notifications on android.

Many errors were repeating, due to differences in British and American English. I standardized the README to American English:
- canceled -> cancelled
- canceling -> cancelling
- informations -> information

Remove many instances of passive/awkward voice:
Before: "See the onadd event of how a listener can be registered to be notified when a local notification has been scheduled."
After: "See the onadd event to register a listener to be notified when a local notification has been scheduled."

Misc:
Many it's and its were confused. Conjunctions were sometimes used at inappropriate times.
Fix possessive apostrophes.

Known Quirks are documented in the README.md

Callbacks for non-repeating notifications were not called if they were not created in the current app instance. Solves #87 partially.