Added 'git2' index management strategy

[Hirevo]

This PR depends on #18.

This PR introduces a new strategy for managing the crate index named git2.
This strategy makes use of the libgit2 library to interact with the crate index repository.
This allows Alexandrie to operate in environments that doesn't have a git command available from the command line.

The strategy's configuration is similar to the 'disk' one:

[index]
type = "git2"        # required
path = "crate-index" # required

The 'git2' strategy is only available if Alexandrie is compiled with the 'git2' feature enabled.
The reason for this feature-gate is to reduce compilation times in case it isn't used/needed to be available.

Yet to do:

• fully implement all Indexer trait methods
• fully document the code (how is it implemented)
• document the feature (at a high-level, how to use, etc...)