Update fieldmanager.php for coding standards

This attempts to move `fieldmanager.php` closer to the WordPress [documentation standards](http://make.wordpress.org/core/handbook/inline-documentation-standards/php-documentation-standards/) and (in a few places) coding standards.

For the most part it rewrites docblocks in accordance with the documentation standards. It tries to correct a few inaccuracies and inconsistencies along the way.

The changes relating to coding standards are mostly for spacing, brackets, and Yoda conditionals.

However, there are also some refactorings that arose from writing the changes:

* In `fieldmanager_set_baseurl()`, use `trailingslashit()` on `$path`, in accordance with the existing documentation.
* In `fm_trigger_context_action()`, bail early if `$calculated_context` is null to avoid doing an action of `fm_`.
* Also in `fm_trigger_context_action()`, use separate `do_action()`s for clarity.

Hopefully I haven't written anything flat-out wrong. I'd be especially interested in feedback about improving the readability of the changes, whether I've misunderstood any PHPDoc guidelines, or whether FM intentionally deviates from the WP standards.