whole schwack of new methods

[doriantaylor]

Resolves #173 and #174.

Created the following new methods:

• .binary? predicate will tell if the input (IO, string, file extension, path, type, whatever) is binary
• #binary? instance method likewise
• .default_type will return either text/plain or application/octet-stream depending on whether the input is perceived to be binary
• #aliases (and .aliases) will return the list of aliases for a given type
• #alias? predicate will determine if a given type is an alias of a canonical type
• #canonical (and .canonical) will return a canonical type for a (potentially aliased) type
• #parents will return the immediate parent types of the given type
• #lineage will return the entire (flattened) inheritance hierarchy of the given type, including itself
• fancy new MimeMagic[…] constructor will generate from a type or file extension

Other changes:

• internal database now contains aliases and the canonical type identifier
• constructor now handles type parameters; params accessor
• explicit #inspect method shows abridged diagnostic representation
• default: keyword parameter on .by_extension, .by_path, .by_magic, .all_by_magic will guarantee a return value when truthy (either the result of .default_type for the given input or a user-supplied value)
• normalized all the documentation to YARD