Started to implement S4 dispatch.

Also added a few more functions to access the properties of a connection.
The implementation was tested with R package 'jsonlite'.

- The reason was the renaming of emthod 'getc' to 'read'. The method is called (by name) from native code.
Implemented option to disable the cache (by setting its size to 0).  This is required for socket connections in order to not block the whole application.
Introduced method 'seekInternal' to be able to call 'invalidateCache' on seeking in base class.

Also added cache invalidation calls when seeking.

Introduced caches for connections which tremendously speeds up little read operations and read operations on compressed files.
However, there is still a problem with Snowfall.

Also added unit tests for this bug.

Now, everything is again byte-based (even in a text-mode connection) and the read bytes are then encoded at once.
This makes the code much cleaner.
However, this assumes that a multi-byte character cannot contain any of the byte signatures of NUL '0x00' or line feed '0x0A'.
But I haven't seen this in the UTF-8/UTF-16 tables.

Fixed issue when setting the r/w seek position in a binary-mode file connection.

Improved implementation of file read-write access; appending was not implemented.
However, currently experiencing some performance issues.