Add check for PHP XSLT Processor extension.

When the PHP XSLT Processor extension is not loaded, Islandora will
silently fail when attempting to add new objects.  This changeset
adds a check in the Islandora hook_requirements() hook for the
XSLTProcessor class.  If it isn't found, a message is thrown.

Conflicts:

	islandora.install
Merge pull request #278 from nigelgbanks/7.x-class-check

Add check for PHP XSLT Processor extension.
Add mapping for dng extension.
Merge pull request #282 from adam-vessey/7.x

Add mapping for dng extension.
Prevent a recursion loop early.
Merge pull request #283 from adam-vessey/7.x

Prevent a recursion loop early.
islandora_delete_object() now follows function description.

islandora_delete_object() no longer throws. Now it will only return TRUE if
successful, FALSE otherwise as expected.

This fixes the issue where solution packs were not reinstalling existing objects.
Added two helper functions.

islandora_user_access_all()
islandora_user_access_any()

They simplify checking for multiple permissions, either requiring all
or only one permission.
Merge pull request #286 from nigelgbanks/7.x-access-helpers

Added two helper functions.
Added the ability to preview a print page for a given object.
Merge pull request #281 from nigelgbanks/7.x-print-menu

Added the ability to preview a print page for a given object.
Merge pull request #284 from nigelgbanks/7.x-solution-pack-issues

islandora_delete_object() now follows function description.
No longer throw exceptions on invalid configuration.
Some additional helper functions.

islandora_is_valid_namespace()
islandora_get_allowed_namespaces()
islandora_get_content_models()
Merge pull request #285 from nigelgbanks/7.x-collection-changes

7.x collection changes
Added in the Fedora_ingest permission to show the manage tab
Merge pull request #288 from philsogaDGI/7.x

Added in the Fedora_ingest permission to show the manage tab
Merge remote branch 'upstream/7.x' into flvc