Every file should have a license type

Since we introduce repository api to moodle 2.0, some imported file may be used illegally.

To solve this problem, we should add a field in mdl_files table to define a license type for each file and a new database table to store different license types.

• Add a `license` field to {files}

  table

• Create a {license}

  table, we store a short name in database, and full name and full content of license should be in language file (license.php)

• API changes
  • attach license information when storing file into moodle file pool
  • return license information when geting file from moodle file pool

• File picker changes
  File picker should have options to choose file license type when getting file from external repository