Add introspection generation

We use the newly added --identifier-filter-cmd option of the
introspection scanner to ensure that the introspection machinery knows
how to handle our types.
build: Remove special cased regexp

We can use generic regular expressions for transforming lower case types
into camel cased ones.
build: Use the $(SED) variable

Don't hardcode `sed`, in case it's not in $PATH.
Merge pull request #10 from ebassi/introspection

Add introspection data to Graphene