Creates interface for http header classes
Changes `fieldName:fieldValue` to `key:value`
Implements basic HTTP header class;

* Returns object of type `header` with attributes `key` and `value`;
* Generates object from header string
* Generates string from header object
* Validates for appropriate format of key and value
Adds function to check if headers have been sent already
`Header::isSent()` returns false if headers not sent
Adds function to set headers
Fixes minor typo
`Cache` class with basic functions to set different headers
Adds function to prevent caching
Adds function to set `Etag` and generate `Etag` from content
Adds function to check and intelligently set etag headers
Adds comments to HeaderInterface
Changes default expires offset to 1 hour for Etag
Adds inline comments in Cache and Header classes
Adds return values to functions that set headers
Merge branch 'master' into cache
Converts tabs to spaces
Adds function to set `Last-Modified` header; Patches 304 header
Changes function name `checkEtagHit` to `doesGivenEtagMatchHttpEtag`
`Header::isSent()` now returns true/false and doesn't throw exceptions