Add offset alignment support

Add offset alignment support

This patch adds a new item which is meant to update the current offset
so that it satisfies a given alignment.

The form is:

    @32
    @64~39
    @16~0xcc

What follows `@` is the alignment value in bits (any multiple of eight
greater than zero). The optional part starting with `~` is the value of
the padding bytes to add to satisfy the alignment requirement.

See the new tests and the `README.adoc` examples to learn more.

`@` used to be an insignificant symbol, but it's not anymore (not
consider as a comment).

Also fixing missing repr() calls at some places.

Change-Id: I9f5ac1da6988c060391640bf2784567a76e9fc48
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>