improve error message for PHP < 5.4

When the user uses a PHP version smaller than 5.4, they are not able
to use the Symfony Installer. Instead, they are presented a nice error
message telling them to use the `composer create-project` command
instead. However, if that command is executed without a version
number, the user will get the latest stable release of the Symfony
Standard Edition. Especially when the user passed a custom version
number to the Symfony Installer, this might not be the desired
version. Therefore, it is reasonable to pass the required version
number to Composer.
Integrated Distill library
feature #38 Integrated Distill library (javiereguiluz)

This PR was squashed before being merged into the 1.0-dev branch (closes #38).

Discussion
----------

Integrated Distill library

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #20
| License       | MIT
| Doc PR        | -

The installer now integrates the [Distill library](https://github.com/raulfraile/distill) to download the best format (ZIP or TGZ) and to uncompress it using the native operating system commands:

![symfony-installer](https://cloud.githubusercontent.com/assets/73419/5088468/330ba72c-6f34-11e4-9fdc-89e462dd296b.gif)

Commits
-------

525370e Integrated Distill library
Improve "About" command contents, which are used as the installer help
feature #40 Improve "About" command contents, which are used as the installer help (javiereguiluz)

This PR was squashed before being merged into the 1.0-dev branch (closes #40).

Discussion
----------

Improve "About" command contents, which are used as the installer help

Improved the contents of the "About" command which is used as the installer help

**Before**

![before](https://cloud.githubusercontent.com/assets/73419/5103605/d7989eb2-6fd9-11e4-95fa-5efd7b8a89d7.png)

**After**

![after](https://cloud.githubusercontent.com/assets/73419/5103608/da4d574c-6fd9-11e4-95a2-fa3b878f313a.png)

Commits
-------

75202e9 Improve "About" command contents, which are used as the installer help
feature #35 improve error message for PHP < 5.4 (xabbuh)

This PR was merged into the 1.0-dev branch.

Discussion
----------

improve error message for PHP < 5.4

When the user uses a PHP version smaller than 5.4, they are not able
to use the Symfony Installer. Instead, they are presented a nice error
message telling them to use the `composer create-project` command
instead. However, if that command is executed without a version
number, the user will get the latest stable release of the Symfony
Standard Edition. Especially when the user passed a custom version
number to the Symfony Installer, this might not be the desired
version. Therefore, it is reasonable to pass the required version
number to Composer.

Commits
-------

11a072c improve error message for PHP < 5.4
Switch autoloader from PSR-0 to PSR-4
minor #32 Switch autoloader from PSR-0 to PSR-4 (jmgq)

This PR was squashed before being merged into the 1.0-dev branch (closes #32).

Discussion
----------

Switch autoloader from PSR-0 to PSR-4

Autoloading was changed to PSR-4, as requested in #26.

Commits
-------

89e4b4d Switch autoloader from PSR-0 to PSR-4
Merge remote-tracking branch 'upstream/master'