fix ability to undefine default aliases in zsh

The ability to not define default aliases in the `.git.scmbrc` file
leading to not having those aliases not present worked fine in bash.

However, this seems to still have thrown an error in zsh.  For example,
if you commented out the line:

        git_checkout_alias="gco"

then when sourcing scm_breeze aliases.sh would throw an error.

This applied only to aliases defined with the `_alias()` function and
not those with the `_git_alias()` function.

To fix this, I wrapped `_alias()` in a similar check to the one
`_git_alias()` has, where it passes the portions as arguments and
verifies the first is present before proceeding.

I'm not 100% certain why seperate functions exist for these two things
to begin with (as far as I can tell from a cursory examination the main
difference is `_git_aliases()` defines a tab completion?) but I believe
my change has fixed the ability to comment out aliases in `.git.scmbrc`
in zsh for users who do not want them (I'm in this boat, there are too
many for me and I get confused).

I tested manually in both bash and zsh and it appears to work, however
you might want to review this before merging given my relative
unfamiliarity with the codebase.
aliases: consistent indentation and quotation

this is unrelated to the previous commit and zero functionality but
while I was changing things I wanted to clean up the indentation and
use consistent single quotes versus double quotes in the functions
calls for `_alias()` and `_git_alias()`.

In my opinion this makes the file more readable, but if it bothers you
let me know and I’ll be happy to remove it from the pull request.
Merge pull request #151 from mroth/zsh_undefine_alias_fix

fix ability to undefine default aliases in zsh