- Added the initial `install_git` command that allows one to install
  direclty from a git repository.

- Added the documentation page for the `install_git` command as well.

- Adjusted code to reflect peer-review from Hadley
- Added the `git_binary` variable to the documentation.

Changed the name of `get_git_version` to `git_version` to be more similar to `git_path` and other nomenclature.
- Removed superfluous comma at the end of `zzz.r` (Thanks @wch)
- Changes:
  - Clone with --depth 1 and --no-hardlinks. Makes it a whole lot faster.
  - Sys.chmod(755) instead of 777
  - Removed the .git from the name we display when checking out the code
    if a `.git` is present in the base name.
  - Removed the `git_version` function as there's no need for it just
    yet.
Require packages in depends.
Fixes #161, #178, #192
Merge pull request #210 from hadley/depends

Require packages in Depends. Fixes #161, #178, #192.
Add system.file shim
Add tests for shim code
Update load_all help with info on shims
Update NEWS
Merge pull request #213 from wch/shim

Replacement for system.file. Fixes #179
Added a much cleaner \.git substitution.
install_bitbucket: make sure repo name is lowercase
Update NEWS
run_onload: call .onLoad with package name
Update NEWS
Re-roxygenize
Add with_lib function
Re-roxygenize
Update NEWS
Merge pull request #215 from wch/with-lib

Add with_lib function. Fixes #183
Merge pull request #169 from davidcoallier/install_git

Ability to install directly from a Git repository URL