Implement a check for dependencies in mat2

Example use:

```
$ mat2 -c
Dependencies required for MAT2 0.1.3:
- Cairo: yes
- Exiftool: yes
- GdkPixbuf from PyGobject: yes
- Mutagen: yes
- Poppler from PyGobject: yes
- PyGobject: yes
```

This should close #35

libmat2/__init__.py

@@ -1,5 +1,13 @@
 #!/bin/env python3
 
+import os
+import collections
+import importlib
+from typing import Dict
+
+# make pyflakes happy
+assert Dict
+
 # A set of extension that aren't supported, despite matching a supported mimetype
 UNSUPPORTED_EXTENSIONS = {
     '.asc',
@@ -17,3 +25,28 @@ UNSUPPORTED_EXTENSIONS = {
     '.xsd',
     '.xsl',
     }
+
+DEPENDENCIES = {
+    'cairo': 'Cairo',
+    'gi': 'PyGobject',
+    'gi.repository.GdkPixbuf': 'GdkPixbuf from PyGobject',
+    'gi.repository.Poppler': 'Poppler from PyGobject',
+    'mutagen': 'Mutagen',
+    }
+
+def check_dependencies() -> dict:
+    ret = collections.defaultdict(bool)  # type: Dict[str, bool]
+
+    exiftool = '/usr/bin/exiftool'
+    ret['Exiftool'] = False
+    if os.path.isfile(exiftool) and os.access(exiftool, os.X_OK):  # pragma: no cover
+        ret['Exiftool'] = True
+
+    for key, value in DEPENDENCIES.items():
+        ret[value] = True
+        try:
+            importlib.import_module(key)
+        except ImportError:  # pragma: no cover
+            ret[value] = False  # pragma: no cover
+
+    return ret