Builds CabalInspector and ModuleInspector with cabal-install using a sandbox

`cabal-install` is used to build `CabalInspector` and `ModuleInspector` (instead of calling `ghc --make`). Moreover, all dependencies are automatically downloaded and installed on a sandbox. This means that the user no longer needs to install `aeson`, `haskell-src-exts` and `haddock` on the user-db (which proves to be problematic in practice), and that the plugin will break less often if ghc is updated (and eventual breakage should be more easily solved by picking the right dependencies in the new .cabal file provided).

This patch subsumes #59 and would close #153.