Edited "Basic Operators" document

Based on the awesome work by @shpakovski in #446. :star2:  

[Rendered Markdown](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/shpakovski-basic-operators/Documentation/BasicOperators.md)

**Major edits:**

 * Added a table of contents.
 * Used subjects for most signal examples.
 * Operators are talked about in terms of "streams" (hopefully clearly), with separate examples for signals and sequences where appropriate.
 * Added `-sequenceNext:` and `-switchToLatest`.
 * Refactored signal combination examples a bit.