solidity_names() recognizes interface

This is a hack. Solidity 0.4.11 introduces the `interface` keyword and
breaks the `solidity_names()` function. This simply tweaks the function
so that it also treats interfaces like contracts and does not break.

I don't believe pyethereum should attempt to parse solidity files on its
own as compatibility issues like this are almost certain to  arise also in the
future. For more long-term a different solution should be found where
pyethereum queries solidity itself and does not parse anything on its own.

ethereum/_solidity.py

@@ -165,6 +165,12 @@ def solidity_names(code):  # pylint: disable=too-many-branches
                 if result:
                     names.append(('contract', result.groups()[0]))
 
+            if char == 'i' and code[pos: pos + 9] == 'interface':
+                result = re.match('^interface[^_$a-zA-Z]+([_$a-zA-Z][_$a-zA-Z0-9]*)', code[pos:])
+
+                if result:
+                    names.append(('contract', result.groups()[0]))
+
             if char == 'l' and code[pos: pos + 7] == 'library':
                 result = re.match('^library[^_$a-zA-Z]+([_$a-zA-Z][_$a-zA-Z0-9]*)', code[pos:])