Created note API

Functionality:
* `note.midi(n: number or string)`: Converts a note in string form (e.g. A#4 or Gb3, will be documented on the wiki) or a given frequency into the respective MIDI code
* `note.freq(n: number or string)`: Converts a note in string form (e.g. A#4) or a given MIDI code into the respective frequency
* `note.get(n: number)`: Converts a MIDI value back into a string; if you have a frequency to convert, just use note.get(note.midi(frequency))
* `note.ticks(n: number)`: Converts note block ticks (0-24) into MIDI code (34-58, respectively) and vice-versa. Useful for use with note blocks and OpenComponents
* `note.play(tone: string or number,duration: number)`: Plays a note from a string or MIDI code via computer.beep
Some tweaks
Update note.lua
Merge pull request #243 from Vexatos/patch-4

Created `note` API to allow note name based use of `computer.beep()`.