file re-organization and enforce coding conventions

No substantial changes. Most of the changes are for coding conventions:

- All ``export``s should be placed in the package file, *i.e.* ``RegERMs.jl``. So that people can look at that file and know what the package provides. (This is the convention of JuliaStats)

- ``RegERMs`` should not contain actual codes. It should only contain exports and a list of included source files. (I moved those codes in ``RegERMs`` to ``optim.jl``)

- Use 4-space indentation instead of tabs. (This is the convention of Julia)

- Names that have already be provided by upstream packages (or Julia base) should be imported to extend (*e.g*, ``optimize`` and ``predict``).

- I change ``classify`` to ``predict`` for models.