Allow 3 digits on Excellon tool selection

Fritzing uses more than 2 digits for tool in their Excellons. To comply
with that, I check specifically for 3 or less digits and use as tool
number, more than that we treat as the standard (2 for tool and 2 for
compensation index)

gerber/excellon_statements.py

@@ -234,9 +234,14 @@ class ToolSelectionStmt(ExcellonStatement):
         """
         line = line[1:]
         compensation_index = None
-        tool = int(line[:2])
-        if len(line) > 2:
+
+        # up to 3 characters for tool number (Frizting uses that)
+        if len(line) <= 3:
+            tool = int(line)
+        else:
+            tool = int(line[:2])
             compensation_index = int(line[2:])
+
         return cls(tool, compensation_index)
 
     def __init__(self, tool, compensation_index=None):

gerber/tests/test_excellon_statements.py

@@ -111,6 +111,9 @@ def test_toolselection_factory():
     stmt = ToolSelectionStmt.from_excellon('T0223')
     assert_equal(stmt.tool, 2)
     assert_equal(stmt.compensation_index, 23)
+    stmt = ToolSelectionStmt.from_excellon('T042')
+    assert_equal(stmt.tool, 42)
+    assert_equal(stmt.compensation_index, None)
 
 def test_toolselection_dump():
     """ Test ToolSelectionStmt to_excellon()