Moved admin

Moved admin functions from 'includes' directory to 'admin' directory,
and renamed main admin file. Removed 'includes' directory.
Moved files to root

Moved plugin files out of 'wp-relogo' subdirectory and into root.
Removed 'wp-relogo' subdirectory.
Validate options input

Added validation for the logo URL to check for a valid protocol and a
.svg file. Bumped plugin to version 0.1.1.
Merge pull request #2 from cconover/dev

Data validation and directory structure