Refactor code and add Composer

[Firesphere]

• Refactored code to PSR-2 standards
• Simplified code where possible
• Removed unused constructors
• Added require_once for all letters
• Added "get words for letter" function
• Added example php file
• Renamed lib to src (use of src is more common than lib)
• Added composer.json for packagist.org
• Changed letter classes to have a static array instead of a method
• Added docblocks
• Changed quotation to " quotes, for easier wording ("yogurt's" versus 'yogurt\'s')
• Aligned arrows of language arrays (This is a personal preference, not a guide or PSR standard)
• Added "count for letter" method
• Added way to get American and British spellings for words starting with a letter separately
• ONLY Supports PHP7.1 (return types etc.)
• Added generated docs from PHP Docblocks

note
Some changes to md files are due to auto formatting. They don't affect the actual output, only readability.