Rename merge shortcode callback function
Refactor `MagicAmazingSystemPlugin::shortcode_merge_handler()`

There are a few major points in this refactor:
- Remove `extract()` and explicitly grab needed variables
- Add the 3rd parameter to `shortcode_atts` (added in WordPress 3.6)
- Clean up logic and extract method stubs for "magic" fields

[Addresses: #25]
[Addresses: #20]
Add method `MagicAmazingSystemPlugin::get_full_name()`
Add method `MagicAmazingSystemPlugin::get_firstname()`
Add method `MagicAmazingSystemPlugin::get_lastname()`
Add 'Last Name' to TinyMCE dropdown
Fix 1ShoppingCart merge tags
Fix email link

Previously the email link was shortening first and last names to just
'Name'. Now, 'Name' expands to 'mc-firstname' and 'mc-lastname'.

Special consideration was required for backwards compatibility for
those who were using 'mc-firstname' in the shortcode as opposed to
just 'firstname'. The same applies to 'mc-lastname'.

[Addresses: #25]
Merge pull request #26 from gMagicScott/feature/refactor-merge-shortcode

Feature/refactor merge shortcode