Split files, add dependencies and remove copied/duplicate code

This was a bigger refactor of the code.
Changes:
1. Removed `defmacro!` related code: imported code from [defmacro-enhance](https://github.com/mabragor/defmacro-enhance/), which took the code from the book, and made a more portable version of the code-walking. It is actively maintained and used by [many projects](http://quickdocs.org/defmacro-enhance/). There's a difference in usage (as described in the `defmacro-enhance`'s README): `g!symbols` becomes `g!-symbols`, and `o!symbols` don't map to `g!symbols`: `o!symbols` becomes `o!-symbols` and map to `o!-symbols`;
2. Code from Paul Graham's On Lisp (like `symb` or `flatten`) was removed and imported from the [source](https://github.com/DalekBaldwin/on-lisp);
3. Removed the `#>` reader-macro: changed to [cl-heredoc](https://github.com/e-user/cl-heredoc) (as described in the [errata](http://letoverlambda.com/index.cl/errata));
4. Kept the [named-readtables](http://common-lisp.net/project/named-readtables/) support;
5. Added the file `readtable.lisp`, with all the read-macro related stuff, with a package called `let-over-lambda.readtable` (with a nickname of `lol.rt`);
6. Added the line `(cl-reexport:reexport-from :lol.rt)` at the end of the `package.lisp` file: it uses the [cl-reexport](https://github.com/takagi/cl-reexport) package and reexports from the `lol` package all symbols from `lol.rt`. That way, one can `(:import-from :let-over-lambda ...)` without having to know if the symbol is in the `lol` package or in the `lol.rt` package.

TODO:
1. Wait for the [on-lisp](https://github.com/DalekBaldwin/on-lisp) package to become available in Quicklisp (I already created an [issue](https://github.com/DalekBaldwin/on-lisp/issues/1), and hope that @DalekBaldwin will agree);
2. Add some tests and check implementation independence (I can do it);
3. Update the README, explaining the API (I can do it too =p );
4. Add docstrings to functions and macros (And this too =p );
5. Test the `if-match` macro (I can try it)