Changed add-,cancel-, and clear function
Revised class-structure

fixed iOS 8.1 build fails with linker error #365

There was a copy paste error in the plugin.xml. That's why 2 files were not compiled.

Added:
Ability to schedule, clear and cancel multiple notifications at once
Ability to update existing notifications
Clear methods on JavaScript-side

Edit README to fix grammar and standardize English

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.