Fix doc tests for libs with deps

The movement of tests to the main `target` directory forgot to update a spot
where rustdoc used a -L flag.
Print help message if cargo is invoked with no arguments
auto merge of #412 : ScriptDevil/cargo/default-help, r=alexcrichton

Most modern command line utilities print usage/help when invoked without any arguments. 

cargo (prior to this patch) defaults to saying "no such subcommand" when invoked without arguments. 
> $ cargo
> No such subcommand

With this patch, it will instead print the help message
> $ cargo
> Rust's package manager
>
> Usage:
> ...
auto merge of #410 : alexcrichton/cargo/fix-doctest-with-dep, r=wycats

The movement of tests to the main `target` directory forgot to update a spot
where rustdoc used a -L flag.

Closes #411
Add all docs in-tree
auto merge of #415 : alexcrichton/cargo/in-tree-docs, r=wycats

There's a bunch of travis config now to rebuild the website whenever something is merged into master.
Update guide.md to use "language: rust" in .travis.yml

travis-ci now supports "language: rust", which eliminates the need to call rustup.sh in .travis.yml. See http://www.reddit.com/r/rust/comments/2ecocp/travis_ci_supports_rust/ for details.
auto merge of #428 : garrison/cargo/patch-3, r=alexcrichton

travis-ci now supports "language: rust", which eliminates the need to call rustup.sh in .travis.yml. See http://www.reddit.com/r/rust/comments/2ecocp/travis_ci_supports_rust/ for details.

This is the same change as #425, but on the master branch in guide.md, not guide.html.
Use `mod` where possible

Example:

```
use std::fmt;
use std::fmt::Show;
```

becomes

```
use std::fmt::{mod, Show};
```
auto merge of #430 : aochagavia/cargo/mod, r=alexcrichton

Example:

```
use std::fmt;
use std::fmt::Show;
```

becomes

```
use std::fmt::{mod, Show};
```
Add a new CARGO_MANIFEST_DIR environment variable

All subprocesses will now be invoked with CARGO_MANIFEST_DIR pointing at the
root of the source directory that they are working on (compiling). This commit
also reorganizes the version environment variables to use the new
infrastructure.

Closes #433
auto merge of #434 : alexcrichton/cargo/issue-433, r=huonw

All subprocesses will now be invoked with CARGO_MANIFEST_DIR pointing at the root of the
source directory that they are working on (compiling). This commit also
reorganizes the version environment variables to use the new infrastructure.

Closes #433
Don't run doc tests on `cargo bench`
Run doc tests over the README if present

The default path to the readme is `README.md`, and this can be overridden with
the `readme` key in the project manifest. A value of `false` will disable
documentation tests, while a string value will be considered the relative path
to the readme.