Refactor

This refactors a huge chunk of the code base to make it more maintainable.

Main points:

* Proper separation between bin and lib. Bin handles argument parsing &
  validation and (most of) the output. Lib provides interfaces for all
  opreations.

* Before, libgit2 internals were literred throughout the codebase,
  mainly the `Repository` struct and `git2::Error` in Results. They
  library is now properly wrapped in `repo.rs`, which exposes only the
  required functionality. It also standardizes the Error messages
  (they're just Strings for now) and handles stuff like the copious
  usage of Options to wrap maybe-invalid-utf-8 values. The program will
  still panic on non-utf-8 Strings e.g. in git remotes, but I guess this
  is acceptable. If you actually manage to hit this case, I promise I'll
  fix it :D

* Many unwraps() are now gone and properly handled.

* The table printing functionality is now confined to `table.rs`,
  instead of passing tables as parameters through the whole program.

Cargo.toml

@@ -32,7 +32,7 @@ path = "src/lib.rs"
 
 [[bin]]
 name = "grm"
-path = "src/main.rs"
+path = "src/grm/main.rs"
 
 [dependencies]