Described the IncomingRequestInterface
Updated IncomingRequestInterface from feedback

Removed:
- setQueryParams() (per discussions on the PR and on-list)
- setIncomingFiles() (per discussions on the PR and on-list)

Added:
- getCookieParams() (per @pmjones on-list)
- getPathParams() (per @pmjones on-list)
- setPathParams() (per @pmjones on-list)
s/getIncomingFiles/getFileParams/

- Both to remove redundancy and for consistency.
Added setCookieParams($cookies) method

- Per https://groups.google.com/d/msgid/php-fig/320aba80-6458-4219-a844-ba8802d6c288%40googlegroups.com
Updated meta document

- Added rationale for the IncomingRequestInterface, including why certain types
  of properties are mutable, and others are not.
- Added information on the "header" debate.
Fix typo

- Reported by @philsturgeon
Merge pull request #338 from weierophinney/feature/incoming-request

[PSR-7] Describe an Incoming Request