Few enhancements

[julienblitte]

Using your great project as back-end with my own GUI, therefore I adapted a bit as bellow:
keepassphp-cli.php:

• more explicit command line
• fix 'rem' command issue: when kbdx is added without kphpdb password, cannot remove it (unless you know how kphpdb is generated)
• enhance 'pwd' command: adding display value of title / username / url / notes / custom fields
• add 'export' command: export database to kdbx file

keepass/keepassphp.php:

• add method getDatabaseFilename(): give ability to export database directly in kdbx format

keepass/lib/database.php:

• add method getStringField(): access StringField via uuid from database and test if uuid exists in database
• add constant KEY_NOTES

keepass/lib/entry.php:

• add method listCustomFields(): list custom fields of an entry

keepass/lib/group.php:

• add method getStringField(): access StringField via uuid from group and test if uuid exists in group
• add method listCustomFields(): access listCustomFields via uuid from group