file rename
remove GetCheckpoint

The corresponding jlog_get_checkpoint call is in neither the perl nor ruby bindings.
It is in the java binding. This is part of an update where I am removing
the Id struct. The java binding looks like a way to directly wrap the C code,
whereas the ruby and perl bindings abstract away some of the unnecessary low level
details. This one function call would fill in an Id with the current checkpoint
of a subscriber. It'd probably be used in ReadInterval, which will also be removed
shortly. ReadMessage/Read will be the only function calls. Ids will not be manually managed.
removed AlterMode

This function is used to basically chmod the file. The chmod
can be done by readers and writers and it can be done at any time.
That seems not so smart. Also, the perl bindings do not support
alter mode.
move all alter options to inside New

New now includes an Options parameter that sets the file mode, safety and journal
size on creation of the file. This is better than changing it in the middle of
reading/writing (because that is likely not possible). Init is also done inside
of New now because managing New, altering, initing, closing, and newing again
is tedious and best done in one New call. There is no use case to create a file
without initing it.
move all write calls to writer.go

This introduces a new struct Writer that embeds jlog.
All writing type calls now use the Writer struct.

This commit also fixes a few minor too-long lines, a JLOG_ constant
name that still existed, and a redeclaration bug.
move all read calls to reader.go

First, this adds a Reader struct in the same manner as the Writer struct.
This also removes all ReadInterval, ReadMessage and ReadCheckpoint calls
and consolidates them into one master Read function that does the same thing
the perl binding's read function does.
Also adds the rewind, checkpoint and autocheckpoint functions that are in
the perl binding.
removes all vestiges of Id and addsd RawSize doc

Id is no longer necessary with everything managed inside Read and
other Checkpoint functions.
change everything to pointer receivers

The Jlog struct is large. Changing it to a pointer receiver will
cut down on copies. Reader and Writer now embed a Jlog pointer.
change writer to be like reader, fix Write to work for Writer