Improving import/export

I try to fix some "bugs" in this module.
Here what I changed:

- import: checking if term already exists using name AND hierarchy
- import: using file_read_contents() instead of using file handlers
- export: default filename is "taxonomy_VocabularyName.xml"
- export: XML is more human-readable: added indents and line feed
- export: using $_SERVER[] superglobal for browser checking
- displayed information on import process
- displayed warning if non vocabulary is present
- Fixed some help text (export)

Bye